Image encoding device and image decoding device

ABSTRACT

An image encoding device includes a first prediction parameter determination section ( 53 ) for selecting, for each of prediction units belonging to a first group, a prediction parameter from a basic set; a second prediction parameter determination section ( 55 ) for selecting, for each of prediction units belonging to a second group, a prediction parameter from a reduced set (i) including at least a part of the prediction parameter(s) selected by the first prediction parameter determination section ( 53 ) and (ii) is constituted by a prediction parameter(s), the number of which is not more than the number of prediction parameters included in the basic set; and a prediction parameter encoding section ( 243 ) for encoding (i) information indicating which one of prediction parameters is selected by the first prediction parameter determination section ( 53 ) and (ii) information indicating which one of prediction parameters is selected by the second prediction parameter determination section ( 55 ).

TECHNICAL FIELD

The present invention relates to an image encoding device for generatingencoded data by encoding an image. Further, the present inventionrelates to an image decoding device for decoding the encoded datagenerated by use of such an image encoding device.

BACKGROUND ART

Video encoding devices have been used for efficiently transmitting orrecording videos. Examples of a specific video encoding method employedin a video encoding device encompass H.264/MPEG-4 AVC (described in NonPatent Literature 1) and a method employed in KTA software which is ajoint development codec in VCEG (Video Coding Expert Group).

According to such an encoding method, an image (picture) constituting avideo is managed in a hierarchical structure which is constituted by (i)a plurality of slices into which the image is divided, (ii) a pluralityof macro blocks into which each of the plurality of slices is divided,and (iii) a plurality of sub blocks into which each of the plurality ofmacro blocks is divided. The encoding is generally carried out per subblock.

Further, according to such an encoding method, a prediction image isgenerally generated on the basis of a locally-decoded image obtained byencoding/decoding an input image. Difference data between the predictionimage and the input image is encoded. Further, examples of a method ofgenerating a prediction image encompass a method called “inter-frameprediction (inter prediction)” and a method called “intra-frameprediction (intra prediction)”.

In the inter prediction, (i) motion compensation employing a motionvector is applied to a reference image in such a reference frame that anentire frame is decoded, and, as a result, (ii) a prediction image in aprediction target frame is generated. Further, in the inter prediction,it is possible to generate a prediction image by referring to aplurality of reference images. In this case, the prediction image isgenerated by use of such a value that a pixel value of each of theplurality of reference images is multiplied by a weighting factor.

Meanwhile, in the intra prediction, prediction images are sequentiallygenerated in such a manner that a prediction image in a frame isgenerated on the basis of a locally-decoded image in the frame.Specifically, the intra prediction is generally such that (i) one ofprediction directions included in a predetermined prediction direction(prediction mode) group is selected for each of a plurality ofprediction units (e.g., a sub block) constituting a unit region (e.g., amacro block), (ii) a pixel value of a reference pixel in alocally-decoded image is extrapolated in the one of predictiondirections thus selected, and, as a result, (iii) a prediction pixelvalue(s) on a prediction target region is generated.

As described above, a prediction image is generally generated on thebasis of a prediction parameter (such as a motion vector, a weightingfactor, and a prediction mode).

CITATION LIST Non Patent Literature

[Non Patent Literature 1]

ITU-T Recommendation H.264 November 2007 (Publication Date: November in2007)

SUMMARY OF INVENTION Technical Problem

However, in order to generate appropriately a prediction image by avideo decoding device, it is necessary to (i) encode a predictionparameter used in the video encoding device and (ii) transmit theencoded parameter to the video decoding device. For this reason, thereis a problem of an increase in an encoding amount of encoded data due tothe prediction parameter.

For example, in the conventional intra prediction described above, it isnecessary to encode (i) the difference data and (ii) prediction modeinformation indicating which prediction mode is selected with respect toeach of prediction target regions. Accordingly, there is a problem of anincrease in an encoding amount of encoded data due to the predictionmode information.

The present invention is made in view of the problems. An object of thepresent invention is to provide (i) an image encoding device which canreduce, without sacrificing encoding efficiency, an encoding amountnecessary to designate a prediction parameter, and (ii) an imagedecoding device which can decode encoded data generated by such an imageencoding device.

Solution to Problem

In order to attain the object, an image encoding device of the presentinvention, for encoding a difference between an input image and aprediction image, includes: classification means for (i) dividing aprediction image into a plurality of unit regions and (ii) classifying aplurality of prediction units included in each of the plurality of unitregions into a first group and a second group; first selection means forselecting, for each of a plurality of prediction units belonging to thefirst group, a prediction parameter designating how to generate aprediction image, the first selection means selecting the predictionparameter from a basic set constituted by predetermined predictionparameters; second selection means for selecting, for each of aplurality of prediction units belonging to the second group, aprediction parameter designating how to generate a prediction image, thesecond selection means selecting the prediction parameter from a reducedset which (i) includes at least a part of the prediction parameter(s)selected by the first selection means and (ii) is constituted by aprediction parameter(s), the number of which is not more than the numberof prediction parameters included in the basic set; and predictionparameter encoding means for encoding (i) information indicating whichone of prediction parameters is selected, for each of the plurality ofprediction units belonging to the first group, by the first selectionmeans and (ii) information indicating which one of prediction parametersis selected, for each of the plurality of prediction units belonging tothe second group, by the second selection means.

According to the image encoding device having the arrangement describedabove, for each of the plurality of prediction units belonging to thesecond group, a prediction parameter designating how to generate aprediction image is selected from the reduced set which (i) includes atleast a part of the prediction parameter(s) selected by the firstselection means for each of the plurality of prediction units belongingto the first group which is included in the unit region in which thesecond group is included, and (ii) is constituted by a predictionparameter(s), the number of which is not more than the predictionparameters included in the basic set. The information indicating whichone of prediction parameters is selected by the second selection meansis encoded.

Here, generally, a prediction parameter with respect to each of theplurality of prediction units has a correlation with a predictionparameter with respect to another prediction unit located in thevicinity of the prediction unit. For this reason, the predictionparameter selected with respect to each of the plurality of predictionunits belonging to the first group is highly likely to be an optimumprediction parameter with respect to each of the plurality of predictionunits belonging to the second group. That is, the parameter selectedfrom the reduced set is highly likely to be an optimum predictionparameter with respect to each of the plurality of prediction unitsbelonging to the second group. Accordingly, with the arrangementdescribed above, it is possible to encode a prediction parameter withouthaving a reduction in encoding efficiency.

Further, with the arrangement described above, the reduced set includesat least a part of the prediction parameter(s) selected by the firstselection means, and is constituted by a prediction parameter(s), thenumber of which is not more than the number of prediction parametersincluded in the basic set. Accordingly, for each of the plurality ofprediction units belonging to the second group, it is possible to have areduction in encoding amount of information indicating which one ofprediction parameters has been selected.

With the arrangement described above, it is therefore possible to have areduction in encoding amount of information designating a predictionparameter, without having a reduction in encoding efficiency.

Further, an image encoding device of the present invention, for encodinga difference between an input image and a prediction image, includes:selection means for selecting, for each of a plurality of predictionunits, a prediction parameter designating how to generate a predictionimage, the selection means selecting the prediction parameter from areduced set including at least a part of a prediction parameter(s)designating how to generate a prediction image(s) for a predictionunit(s) which (i) is located in the vicinity of a corresponding one ofthe plurality of prediction units and (ii) has been encoded; andprediction parameter encoding means for encoding, for each of theplurality of prediction units, information indicating which one ofprediction parameters has been selected by the selection means.

Generally, a prediction parameter with respect to each of a plurality ofprediction units has a correlation with a prediction parameter withrespect to another prediction unit located in the vicinity of theprediction unit. Accordingly, the reduced set is highly likely toinclude an optimum prediction parameter in generation of a predictionimage of each of the plurality of prediction units. Further, the reducedset is constituted by at least a part of the prediction parameter(s)with respect to the prediction unit(s) which is located in the vicinityof a corresponding one of the plurality of prediction units.Accordingly, the number of prediction parameters included in the reducedset is smaller than the number of prediction parameters included in aparameter set constituted by prediction parameters with respect toprediction units other than the each of the plurality of predictionunits.

According to the image encoding device of the present invention, havingthe arrangement described above, it is therefore possible to generateencoded data whose encoding amount is small, without sacrificingencoding efficiency.

Furthermore, an image decoding device of the present invention, fordecoding encoded data which is obtained in such a manner that (i) adifference between an original image and a prediction image is encodedfor each of a plurality of prediction units, and, simultaneously, (ii)selection information is encoded, the selection information indicatingwhich one of a plurality of prediction parameters, each designating howto generate a prediction image, is selected for each of the plurality ofprediction units, includes: classification means for classifying aplurality of prediction units included in each of a plurality of unitregions constituting the prediction image into a first group and asecond group; first selection means for selecting, for each of theplurality of prediction units belonging to the first group, a predictionparameter designating how to generate a prediction image, the firstselection means selecting, by referring to selection information foreach of a plurality of prediction units belonging to the first group,the prediction parameter from a basic set constituted by predeterminedprediction parameters; and second selection means for selecting, foreach of the plurality of prediction parameters belonging to the secondgroup, a prediction parameter designating how to generate a predictionimage, the second selection means selecting, by referring to selectioninformation for each of a plurality of prediction units belonging to thesecond group, the prediction parameter from a reduced set which (i)includes at least a part of the prediction parameter(s) selected by thefirst selection means and (ii) is constituted by a predictionparameter(s), the number of which is not more than the number of thepredetermined prediction parameters included in the basic set.

According to the image decoding device having the arrangement describedabove, it is possible to select, for each of the plurality of predictionunits belonging to the second group, a prediction parameter designatinghow to generate a prediction image, from the reduced set which (i)includes at least a part of the prediction parameter(s) selected by thefirst selection means for each of the plurality of prediction unitsbelonging to the first group which is included in the unit region towhich the second group belongs, and (ii) is constituted by a predictionparameter(s), the number of which is not more than the number ofprediction parameters included in the basic set.

Here, generally, a prediction parameter with respect to each of theprediction units has a correlation with a prediction parameter withrespect to another prediction unit located in the vicinity of theprediction unit. Accordingly, the prediction parameter selected for eachof the plurality of prediction units belonging to the first group ishighly likely to be an optimum prediction parameter with respect to eachof the plurality of prediction units belonging to the second group. Withthe arrangement described above, it is therefore possible to decode,without having a reduction in encoding efficiency, the predictionparameter from selection information having a reduction in encodingamount.

Moreover, an image decoding device of the present invention, fordecoding encoded data which is obtained in such a manner that (i) adifference between an input image and a prediction image is encoded foreach of a plurality of prediction units, and, simultaneously, (ii)selection information is encoded, the selection information indicatingwhich one of a plurality of prediction parameters, each designating howto generate a prediction image, is selected for each of the plurality ofprediction units, includes: selection means for selecting, for each of aplurality of prediction units, a prediction parameter designating how togenerate a prediction image, the selection means selecting theprediction parameter from a reduced set including at least a part of aprediction parameter(s) designating how to generate a predictionimage(s) for a prediction unit(s) which (i) is located in the vicinityof a corresponding one of the plurality of prediction units and (ii) hasbeen decoded.

Generally, a prediction parameter with respect to each of a plurality ofprediction units has a correlation with a prediction parameter withrespect to another prediction unit located in the vicinity of theprediction unit. Accordingly, the reduced set is highly likely toinclude an optimum prediction parameter in generation of a predictionimage of each of the plurality of prediction units. Further, the reducedset is constituted by at least a part of the prediction parameter(s)with respect to the prediction unit(s) which is located in the vicinityof a corresponding one of the plurality of prediction units.Accordingly, the number of prediction parameters included in the reducedset is smaller than the number of prediction parameters included in aparameter set constituted by prediction parameters with respect toprediction units other than the each of the plurality of predictionunits.

According to an image encoding device having an arrangementcorresponding to the arrangement described above, it is thereforepossible to generate encoded data whose encoding amount is small,without sacrificing encoding efficiency.

The image decoding device having the arrangement described above candecode the encoded data whose encoding amount is small as describedabove.

Further, a data structure of the present invention, for encoded datawhich is obtained in such a manner that (i) a difference between aninput image and a prediction image is encoded for each of a plurality ofprediction units, and, simultaneously, (ii) selection information isencoded, the selection information indicating which one of a pluralityof prediction parameters, each designating how to generate a predictionimage, is selected for each of the plurality of prediction units,includes: selection information which is referred to in an imagedecoding device for decoding the encoded data, so as to select, for eachof the plurality of prediction units, a prediction parameter designatinghow to generate a prediction image, the prediction parameter beingselected from a reduced set which includes at least a part of aprediction parameter(s) designating how to generate a predictionimage(s) for a prediction unit(s) which (i) is located in the vicinityof a corresponding one of the plurality of prediction units and (ii) hasbeen decoded.

Generally, a prediction parameter with respect to each of a plurality ofprediction units has a correlation with a prediction parameter withrespect to another prediction unit located in the vicinity of theprediction unit. Accordingly, the reduced set is highly likely toinclude an optimum prediction parameter in generation of a predictionimage of each of the plurality of prediction units. Further, the reducedset is constituted by at least a part of the prediction parameter(s)with respect to the prediction unit(s) which is located in the vicinityof a corresponding one of the plurality of prediction units.Accordingly, the number of prediction parameters included in the reducedset is smaller than the number of prediction parameters included in aparameter set constituted by prediction parameters with respect toprediction units other than the each of the plurality of predictionunits.

Accordingly, the encoded data having the structure described above isthe encoded data, whose encoding amount is reduced without sacrificingencoding efficiency.

Advantageous Effects of Invention

As described above, an image encoding device of the present invention,for encoding a difference between an input image and a prediction image,includes: classification means for (i) dividing a prediction image intoa plurality of unit regions and (ii) classifying a plurality ofprediction units included in each of the plurality of unit regions intoa first group and a second group; first selection means for selecting,for each of a plurality of prediction units belonging to the firstgroup, a prediction parameter designating how to generate a predictionimage, the first selection means selecting the prediction parameter froma basic set constituted by predetermined prediction parameters; secondselection means for selecting, for each of a plurality of predictionunits belonging to the second group, a prediction parameter designatinghow to generate a prediction image, the second selection means selectingthe prediction parameter from a reduced set which (i) includes at leasta part of the prediction parameter(s) selected by the first selectionmeans and (ii) is constituted by a prediction parameter(s), the numberof which is not more than the number of prediction parameters includedin the basic set; and prediction parameter encoding means for encoding(i) information indicating which one of prediction parameters isselected, for each of the plurality of prediction units belonging to thefirst group, by the first selection means and (ii) informationindicating which one of prediction parameters is selected, for each ofthe plurality of prediction units belonging to the second group, by thesecond selection means.

According to the image encoding device having the arrangement describedabove, it is possible to reduce, without sacrificing encodingefficiency, an encoding amount necessary to designate a predictionparameter.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating an arrangement of a videodecoding device in accordance with an embodiment of the presentinvention.

FIG. 2 is a block diagram illustrating an arrangement of an MB decodingsection included in the video decoding device in accordance with theembodiment of the present invention.

FIG. 3 is a block diagram illustrating an arrangement of a predictionparameter decoding section included in the video decoding device inaccordance with the embodiment of the present invention.

FIG. 4 is an explanatory view showing how a group determination sectionincluded in the prediction parameter decoding section operates: (a) and(b) of FIG. 4 show how 16 sub blocks included in a macro block areclassified into a first group and a second group in accordance with aclassification method A; (c) and (d) of FIG. 4 show how the 16 subblocks are classified into the first block and the second block inaccordance with a classification method B; and (e) and (f) of FIG. 4show how the 16 sub blocks are classified into the first group and thesecond group in accordance with a classification method C.

FIG. 5 is a view showing (i) intra prediction modes used in intraprediction in compliance with an H.264/MPEG-4 AVC standard and (ii)indexes attached to the respective intra prediction modes.

FIG. 6 is an explanatory view showing how a reduced set derivationsection included in the prediction parameter decoding section operates:(a) of FIG. 6 is a flow chart showing a first example of an operation ofgenerating a reduced set by the reduced set derivation section; (b) ofFIG. 6 is a flow chart showing a second example of the operation ofgenerating the reduced set by the reduced set derivation section; and(c) of FIG. 6 is a flow chart showing a third example of the operationof generating the reduced set by the reduced set derivation section.

FIG. 7 is a flow chart showing an example of how a decoding process iscarried out by a second prediction parameter decoding section includedin the prediction parameter decoding section.

FIG. 8 is an explanatory view showing another example of the arrangementof the prediction parameter decoding section: (a) of FIG. 8 is a flowchart showing an operation of generating a reduced set by the reducedset derivation section; and (b) of FIG. 8 shows an example of aproximity sub block region.

FIG. 9 is an explanatory view showing an operation of generating aprediction image by a prediction image generation section included inthe MB decoding section, specifically, showing each of a plurality ofpixels (4×4) of a prediction target sub block and pixels in the vicinityof the prediction target sub block.

FIG. 10 is a block diagram illustrating an arrangement of a videoencoding device in accordance with the embodiment of the presentinvention.

FIG. 11 is a block diagram illustrating an arrangement of an MB encodingsection included in the video encoding device in accordance with theembodiment of the present invention.

FIG. 12 is a block diagram illustrating an arrangement of a predictionparameter determination section included in the MB encoding section.

FIG. 13 is an explanatory view showing how the prediction parameterdetermination section included in the MB encoding section operates: (a)of FIG. 13 is a view showing an example of prediction modes which areselected by a first prediction parameter determination section withrespect to a plurality of sub blocks belonging to a first group, among aplurality of sub blocks constituting a macro block MB; (b) of FIG. 13 isa view showing an example of a reduced set which is generated by areduced set derivation section in a case where the prediction modesshown in (a) of FIG. 13 are supplied as prediction parameters; and (c)of FIG. 13 is a view showing an example of prediction modes which areselected from the reduced set shown in (b) of FIG. 13, with respect to aplurality of sub blocks belonging to a second group, by a secondprediction parameter determination section.

FIG. 14 is a block diagram illustrating an arrangement of a predictionparameter encoding section included in the MB encoding section.

FIG. 15 is a view showing a bit stream structure for each macro block ofencoded data which (i) is generated by the video encoding device inaccordance with the embodiment of the present invention and (ii) isreferred to by the video decoding device in accordance with theembodiment of the present invention.

FIG. 16 is a view showing another example of a basic parameter set: (a)of FIG. 16 is a view showing an example of a parameter set whose maindirection is a horizontal direction; and (b) of FIG. 16 is a viewshowing an example of a parameter set whose main direction is a verticaldirection.

DESCRIPTION OF EMBODIMENTS

(Video Decoding Device)

An arrangement of a video decoding device (image decoding device) 1 inaccordance with an embodiment of the present invention is describedbelow with reference to FIGS. 1 through 9. The video decoding device 1employs, in a part of the video decoding device 1, a technique adoptedin an H. 264/MPEG-4 AVC standard.

To put it shortly, the video decoding device 1 generates a decoded image#2 by decoding encoded data #1 supplied to the video decoding device 1,and outputs the decoded image #2 thus generated.

Further, a video decoding device 1 divides a unit region on an imageindicated by the encoded data #1 into a plurality of prediction targetregions (prediction units), and generates the decoded image #2 by use ofa prediction image generated for each of the plurality of predictiontarget regions.

The following description deals with, as an example, a case where theunit region is a macro block defined in the H. 264/MPEG-4 AVC standard,and the prediction target region is a sub block in the macro block.Note, however, that the present invention is not limited to this. Forexample, the unit region may be a region larger than the macro block, ora region which overlaps a plurality of macro blocks.

FIG. 1 is a block diagram illustrating an arrangement of the videodecoding device 1. The video decoding device 1 includes avariable-length code inverse multiplexing section 11, a headerinformation decoding section 12, an MB setting section 13, an MBdecoding section 14, and a frame memory 15 (see FIG. 1).

The encoded data #1 supplied to the video decoding device 1 is inputtedinto the variable-length code inverse multiplexing section 11. Thevariable-length code inverse multiplexing section 11 inverse-multiplexesthe encoded data #1 to separate the encoded data #1 into (i) headerencoded data #11 a which is encoded data related to header informationand (ii) MB encoded data #11 b which is encoded data related to macroblocks (unit regions). The variable-length code inverse multiplexingsection 11 then outputs the header encoded data #11 a to the headerinformation decoding section 12 and the MB encoded data #11 b to the MBsetting section 13.

The header information decoding section 12 decodes header information#12 from the header encoded data #11 a. Here, the header information #12is information including a size of an input image.

On the basis of the header information #12 thus received, the MB settingsection 13 separates out, from the MB encoded data #11 b, encoded data#13 which corresponds to each of a plurality of macro blocks. Then, theMB setting section 13 successively outputs the encoded data #13 to theMB decoding section 14.

The MB decoding section 14 generates a decoded image #2 corresponding toeach of the plurality of macro blocks by successively decoding theencoded data #13 corresponding to each of the plurality of macro blocks,and then, outputs the decoded images #2. Further, the MB decodingsection 14 also outputs the decoded image #2 to the frame memory 15.Details of an arrangement of the MB decoding section 14 will bedescribed later, and therefore an explanation of the arrangement of theMB decoding section 14 is omitted here.

The decoded image #2 is stored in the frame memory 15. At the time thata certain macro block is decoded, decoded images corresponding to allthe macro blocks which were previously provided in a raster scan orderwith respect to the certain macro block has been stored in the framememory 15.

When the MB decoding section 14 completes a process of generating adecoded image, per macro block, with respect to all the plurality ofmacro blocks in an image, a process of generating the decoded image #2corresponding to the encoded data supplied to the video decoding device1 is completed.

(MB Decoding Section 14)

The following description specifically deals with details of the MBdecoding section 14 with reference to another drawing.

FIG. 2 is a block diagram illustrating an arrangement of the MB decodingsection 14. The MB decoding section 14 includes a sub block dividingsection 141, a prediction residual decoding section 142, a sub blockdecoded image generation section 143, a prediction parameter decodingsection 144, a prediction image generation section 145, and an MBdecoded image generation section 146 (see FIG. 2).

The sub block dividing section 141 is started up when the encoded data#13 (unit: macro block) is supplied to the sub block dividing section141. The sub block dividing section 141 sequentially outputs, in apredetermined order, (i) sub block position information #141 aindicating a position of each of a plurality of sub blocks (predictiontarget regions) in a macro block (unit region) which is constituted bythe plurality of sub blocks, and (ii) sub block encoded data #141 bwhich is encoded data related to the sub block indicated by the subblock position information #141 a. Note that a method used by the videoencoding device for generating the encoded data #1 can be applied to amethod of dividing the macro block into the plurality of sub blocks.

Further, it is preferable that the sub block dividing section 141outputs sub block position information #141 a and sub block encoded data#141 b, both of which are related to each of a plurality of sub blocksbelonging to a first group (later described), and then outputs sub blockposition information #141 a and sub block encoded data #141 b, both ofwhich are related to each of a plurality of sub blocks belonging to asecond group (later described). For example, it is preferable that theplurality of sub blocks belonging to the first group are scanned in araster scan order, and then, the plurality of sub blocks belonging tothe second group are scanned in the raster scan order. Furthermore, itis also possible that the sub block dividing section 141 outputs the subblock position information #141 a and the sub block encoded data #141 bin an order which is identical with an order used by the video encodingdevice for generating the encoded data #1.

The prediction residual decoding section 142 applies variable-lengthencoding/decoding to the sub block encoded data #141 b thus inputted, soas to generate a transform coefficient with respect to the sub blockindicated by the sub block position information #141 a thus inputted.Further, the prediction residual decoding section 142 applies, to thetransform coefficient thus generated, inverse transformation of DCT(Discrete Cosine Transform) having the same size as that of the subblock indicated by the sub block position information #141 a, so as togenerate a decoded residual #142. Then, the prediction residual decodingsection 142 outputs the decoded residual #142.

The prediction parameter decoding section 144 decodes, in accordancewith the sub block position information #141 a and the sub block encodeddata #141 b, a prediction parameter #144 with respect to each of theplurality of sub blocks, and then, outputs the prediction parameter#144.

Here, the prediction parameter is a parameter used to generate aprediction image. Examples of the prediction parameter encompass aprediction mode in intra prediction, a motion vector in motioncompensation prediction, and a weighting factor in luminancecompensation prediction.

Moreover, the prediction parameter #144 includes a prediction parameter#43 outputted from a first prediction parameter decoding section 43(later described) and a prediction parameter #45 outputted from a secondprediction parameter decoding section 45 (later described). Details ofan arrangement of the prediction parameter decoding section 144 anddetails of how the prediction parameter decoding section 144 operateswill be described later, and therefore explanations of these are omittedhere.

In accordance with the prediction parameter #144, the decoded image #2,and a decoded image #15 stored in the frame memory 15, the predictionimage generation section 145 generates a prediction image #145corresponding to the prediction target sub block. Then, the predictionimage generation section 145 outputs the prediction image #145. Aspecific method of generating the prediction image #145 by theprediction image generation section 145 will be described later, andtherefore an explanation of the method is omitted here.

The sub block decoded image generation section 143 adds the predictionimage #145 to a decoded residual #142, so as to generate a sub blockdecoded image #143 whose unit is a sub block. Then, the sub blockdecoded image generation section 143 outputs the sub block decoded image#143.

The MB decoded image generation section 146 accumulates, for each of theplurality of macro blocks, the sub block decoded images #143 whose unitis a sub block, and integrates, with each other, all the sub blockdecoded images #143 constituting the macro block. The MB decoded imagegeneration section 146 thus generates a decoded image #2 whose unit is amacro block, and then outputs the decoded image #2. The decoded image #2thus generated is also supplied to the prediction image generationsection 145.

(Prediction Parameter Decoding Section 144)

Next, the following description deals with the arrangement of theprediction parameter decoding section 144 with reference to FIG. 3.

FIG. 3 is a block diagram illustrating the arrangement of the predictionparameter decoding section 144. The prediction parameter decodingsection 144 includes a group determination section 41, a switch section42, the first prediction parameter decoding section 43, a reduced setderivation section 44, and the second prediction parameter decodingsection 45 (see FIG. 3).

(Group Determination Section 41)

The group determination section 41 determines which group the sub blockindicated by the sub block position information #141 a belongs to, amonga plurality of predetermined groups. Then, the group determinationsection 41 outputs, to the switch section 42, group information #41indicating a result of the determination.

Here, the plurality of predetermined groups may be, for example, aplurality of groups into which a plurality of sub blocks have beenclassified in the video encoding device for generating the encoded data#1. That is, in the video encoding device for generating the encodeddata #1, each of sub blocks SB1 through SBNs (Ns is a total number ofthe sub blocks belonging to a macro block MB) belonging to a certainmacro block MB is classified into a corresponding one of groups GP1through GPM (M is a total number of groups into which the sub blocksbelonging to the macro block MB are classified) in accordance with apredetermined classification method. In a case where a sub block SBn isclassified into a group GPm, the group determination section 41determines, on the basis of the predetermined classification method, forexample, that the sub block SBn indicated by the sub block positioninformation #141 a belongs to the group GPm.

The following description deals with an example of how sub blocks areclassified into 2 groups, with reference to (a) through (f) of FIG. 4.

(a) and (b) of FIG. 4 show how 16 sub blocks included in a macro blockMB are classified into a first group and a second group in accordancewith a classification method A. (c) and (d) of FIG. 4 show how the subblocks are classified in accordance with a classification method B. (e)and (f) of FIG. 4 show how the sub blocks are classified in accordancewith classification method C.

The sub blocks included in the macro block MB may be classified into thefirst group and the second group so that (i) the sub blocks included inthe first group and the sub blocks included in the second group arearranged in a checkered flag pattern, as shown in (a) and (b) of FIG. 4,(ii) the sub blocks included in the first group are adjacent to eachother only in a horizontal direction, and the sub blocks included in thesecond blocks are adjacent to each other only in the horizontaldirection, as shown in (c) and (d) of FIG. 4, or (iii) the sub blocksincluded in the first group are adjacent to each other only in avertical direction, and the sub blocks included in the second group areadjacent to each other only in the vertical direction, as shown in (e)and (f) of FIG. 4.

Generally, an optimum classification method differs in accordance with aspatial correlation between prediction parameters in the macro block.The aforementioned classification method A is effective in a case wherea vertical spatial correlation or a horizontal spatial correlationexists. Meanwhile, in a case where there is an edge in an obliquedirection in the macro block, the aforementioned classification methodsB and C are more effective than the classification method A.

With any one of the aforementioned classification methods, the subblocks are classified into the first group and the second group inaccordance with positions of the sub blocks in the macro block, as isclear from (a) through (f) of FIG. 4.

The group determination section 41 refers to the sub block positioninformation #141 a, and determines, on the basis of the classificationmethod used by the video encoding device for generating the encoded data#1, which one of the first group and the second group the sub blockindicated by the sub block position information #141 a belongs to.

For example, in a case where the video encoding device for generatingthe encoded data #1 classifies, on the basis of the classificationmethod A, the sub block SB1 belonging to the macro block MB into thesecond group, and the sub block SB2 into the first group (as shown in(a) and (b) of FIG. 4), the group determination section 41 determines,by referring to the sub block position information #141 a, on the basisof the classification method A, that the sub block SB2 belongs to thefirst group and the sub block SB1 belongs to the second group. As to theother sub blocks included ion the macro block MB, the groupdetermination section 41 carries out the determination in the samemanner as the sub blocks SB1 and SB2.

Further, in a case where the video encoding device for generating theencoded data #1 employs different classification methods for respectivemacro blocks, it is preferable that the encoded data #1 includes a flagindicating which one of classification methods has been used for acorresponding macro block. By referring to such a flag, the groupdetermination section 41 can carry out the determination on the basis ofthe classification method used by the video encoding device, even in acase where different classification methods are used for respectivemacro blocks.

In the above explanations, the number of sub blocks included in themacro block is 16. Note, however, that the present invention is notlimited to this arrangement (this also applies to the following cases).Further, how to classify the sub blocks in the macro block is notlimited to the aforementioned classification methods, and otherclassification methods can be employed. For example, it is possible toemploy such a classification method that the number of the sub blocksbelonging to the first group and the number of the sub blocks belongingto the second group are different from each other (this also applies tothe following cases).

(Switch Section 42)

On the basis of the group information #41, the switch section 42transmits, to one of the first prediction parameter decoding section 43and the second prediction parameter decoding section 45, the sub blockencoded data #141 b which is encoded data related to the sub blockindicated by the sub block position information #141 a.

Specifically, in a case where the group determination section 41determines that the sub block indicated by the sub block positioninformation #141 a belongs to the first group, the switch section 42transmits the sub block encoded data #141 b to the first predictionparameter decoding section 43. On the other hand, in a case where thegroup determination section 41 determines that the sub block indicatedby the sub block position information #141 a belongs to the secondgroup, the switch section 42 transmits the sub block encoded data #141 bto the second prediction parameter decoding section 45.

(First Prediction Parameter Decoding Section 43)

The first prediction parameter decoding section 43 decodes the sub blockencoded data #141 b so as to decode the prediction parameter #43, whichhas been used by the video encoding device for generating the encodeddata #1 to predict the sub block (prediction target sub block) indicatedby the sub block position information #141 a. Then, the first predictionparameter decoding section 43 outputs the prediction parameter #43.

More specifically, the first prediction parameter decoding section 43first sets, as an estimate value with respect to the prediction targetsub block, the prediction parameter which (i) has been used inprediction of a sub block located on an upper side (or on a left side)with respect to the prediction target sub block, and (ii) has beendecoded.

Next, the first prediction parameter decoding section 43 decodes a flagincluded in the sub block encoded data #141 b.

In a case where the flag indicates that the estimate value is to beused, the first prediction parameter decoding section 43 sets theestimate value as the prediction parameter with respect to theprediction target sub block. On the other hand, in a case where the flagindicates that the estimate value is not to be used, the firstprediction parameter decoding section 43 sets a prediction parameterdecoded from a part other than a part of the flag, as the predictionparameter with respect to the prediction target sub block.

In a case where the sub block located on the upper side (or on the leftside) with respect to the prediction target sub block has not beendecoded, the first prediction parameter decoding section 43 may referto, as the estimate value, a prediction parameter used in prediction ofa sub block which (i) is located on the upper side (or on the left side)with respect to the prediction target sub block, (ii) has been decoded,and (iii) is located in a position closest to the prediction target subblock among the sub blocks which have been decoded.

Note that the prediction parameter #43 thus decoded is also supplied tothe reduced set derivation section 44.

With the operations described above, the reduced set derivation section44 is supplied with the prediction parameter #43 decoded from each ofthe plurality of sub blocks belonging to the first group.

(Reduced Set Derivation Section 44)

The reduced set derivation section 44 accumulates the predictionparameters #43, so as to generate a reduced prediction parameter set RS(hereinafter, referred to as “reduced set RS”). Here, the reduced set RSis a set including the prediction parameter #43 decoded from each of theplurality of sub blocks belonging to the first group. Further, thereduced set RS may include another prediction parameter other than theprediction parameter #43.

Furthermore, in a case where the same parameter is decoded from aplurality of sub blocks, among the plurality of sub blocks belonging tothe first group, the reduced set derivation section 44 generates such areduced set RS that one prediction parameter is included for theplurality of sub blocks from which the same parameter is decoded. Inother words, the reduction set derivation section 44 generates thereduced set RS so that there are no prediction parameters which areidentical with each other in the reduced set RS. For example, in a casewhere, among the sub blocks SB1 through SB16 belonging to the firstgroup, a prediction parameter PP1 is decoded from each of the sub blocksSB1 through SB8, and a prediction parameter PP2 is decoded from each ofthe sub blocks SB9 through SB16, the reduced set derivation section 44generates a reduced set RS which includes one prediction parameter PP1and one prediction parameter PP2.

The following description deals with an example in which a predictionparameter is an intra prediction mode defined in the H.264/MPEG-4 AVCstandard. How the reduced set derivation section 44 generates a reducedset RS is described below with reference to FIG. 5, and (a) through (c)of FIG. 6.

FIG. 5 is a view showing (i) intra prediction modes (hereinafter,referred to as “prediction modes”) used in the intra prediction definedin the H.264/MPEG-4 AVC standard and (ii) an index attached to each ofthe prediction modes. Each of the intra prediction modes indicates aprediction direction used in the intra prediction. According to the H.264/MPEG-4 AVC standard, prediction modes of 8 directions (correspondingto indexes of 0, 1, and 3 through 8), and a DC prediction mode(corresponding to an index of 2) are used (see FIG. 5). Hereinafter, aprediction mode designated with an index I is referred to as “predictionmode I”. Further, a parameter set constituted by the prediction modes 0through 8 is referred to as “basic parameter set”.

(Example 1 of Generation of Reduced Set RS)

(a) of FIG. 6 is a flow chart showing a first example of how the reducedset derivation section 44 generates a reduced set RS.

As shown in (a) of FIG. 6, first, the reduced set derivation section 44sets the reduced set RS to be empty, so as to initialize the reduced setRS (Step S101).

Next, the reduced set derivation section 44 adds, to the reduced set RS,the prediction parameter #43 decoded from each of the plurality of subblocks belonging to the first group (Step S102). For example, in a casewhere a prediction mode 1, a prediction mode 6, and a prediction mode 8are decoded from the plurality of sub blocks belonging to the firstgroup, the reduction set derivation section 44 adds the prediction mode1, the prediction mode 6, and the prediction mode 8 to the reduced setRS.

According to the first example, with the operations described above, itis possible for the reduced set derivation section 44 to generate thereduced set RS constituted by the prediction parameter #43 decoded fromeach of the plurality of sub blocks belonging to the first group.

Generally, there is a correlation between optimum prediction parameterswith respect to a plurality of sub blocks constituting a macro block.Accordingly, the prediction parameters selected with respect to theplurality of sub blocks belonging to the first group are highly likelyto be optimum prediction parameters with respect to the plurality of subblocks belonging to the second group. Further, the number of predictionmodes included in the reduced set RS is smaller than the number ofprediction parameters included in the basic parameter set.

Accordingly, the video encoding device for generating the encoded data#1, having an arrangement corresponding to the arrangement of thepresent example, can generate, without sacrificing encoding efficiency,the encoded data #1 whose encoding amount is small. Further, the videodecoding device 1 having the arrangement of the present example candecode the encoded data #1 thus generated, whose encoding amount issmall.

(Second Example of Generation of Reduced Set RS)

(b) of FIG. 6 is a flow chart showing a second example of how thereduced set derivation section 44 generates a reduced set RS.

As shown in (b) of FIG. 6, the reduced set derivation section 44 firstsets the reduced set RS to be empty, so as to initialize the reduced setRS (Step S201).

Next, the reduced set derivation section 44 adds an additional parameterset AS to the reduced set RS (Step S202). Here, it is preferable thatthe additional parameter set AS includes a prediction parameter(s) whichtends to be used frequently. According to the H.264/MPEG-4 AVC standard,generally, the smaller an index designating a prediction mode is, themore frequently the prediction mode tends to be used in the intraprediction. Accordingly, it is preferable that the additional parameterset AS includes a prediction mode designated with a small index amongthe indexes 0 through 8. For example, it is preferable that theadditional parameter set AS includes the prediction mode 0 (the verticaldirection prediction mode), the prediction mode 1 (the horizontaldirection prediction mode), and the prediction mode 2 (the DC predictionmode).

Further, it is also possible that the additional parameter set ASincludes at least one of the prediction mode 0, the prediction mode 1,and the prediction mode 2.

Next, the reduced set derivation section 44 adds, to the reduced set RS,the prediction parameter #43 decoded from each of the plurality of subblocks belonging to the first group (Step S203). Note, however, that itis preferable that, among the prediction parameter #43, the reduced setderivation section 44 does not add, to the reduced set RS, theprediction parameter which has been already included in the reduced setRS. That is, it is preferable that the reduced set RS include noprediction parameters identical with each other. For example, in a casewhere the prediction mode 1 and the prediction mode 4 is obtained as theprediction parameter #43, and the reduced set RS has already includedthe prediction mode 1 and the prediction mode 2, the reduced setderivation section 44 adds only the prediction mode 4 to the reduced setRS.

According to the second example, with the operations described above, itis possible for the reduced set derivation section 44 to generate thereduced set RS which is constituted by (i) the prediction parameter #43decoded from each of the plurality of sub blocks belonging to the firstgroup and (ii) the prediction mode(s) included in the additionalparameter set.

By constituting the reduced set RS as described above, it is possible togenerate the reduced set RS which is constituted by (i) the predictionparameter #43 decoded from each of the plurality of sub blocks belongingto the first group and (ii) the prediction mode(s) which tends to beused frequently.

Accordingly, the video encoding device for generating the encoded data#1, including a reduced set derivation section which operates as in thepresent example, can generate such encoded data #1 that encoding amountof a prediction residual is small. Furthermore, the video decodingdevice 1 includes the reduced set derivation section 44 which operatesas in the present example can decode such encoded data #1 that encodingamount of the prediction residual is small.

(Third Example of Generation of Reduced Set RS)

(c) of FIG. 6 is a flow chart showing a third example of how the reducedset derivation section 44 generates a reduced set RS.

As shown in (c) of FIG. 6, the reduced set derivation section 44 firstsets the reduced set RS to be empty, so as to initialize the reduced setRS (Step S301).

Next, the reduced set derivation section 44 adds, to the reduced set RS,the prediction parameter #43 decoded from each of the plurality of subblocks belonging to the first group (Step S302).

Then, the reduced set derivation section 44 determines whether or not“log₂ (Np−1)” is an integer (Step S303). Here, “Np” is the number ofprediction parameters included in the reduced set RS.

In a case where “log₂ (Np−1)” is an integer (Yes in Step S303), thereduced set derivation section 44 outputs the reduced set RS.

In a case where “log₂ (Np−1)” is not an integer (No in Step S303), thereduced set derivation section 44 adds a predetermined predictionparameter to the reduced set (Step S304), and carries out the process ofStep S303 again. Here, the predetermined prediction parameter is, forexample, a prediction mode selected from the prediction modes 0 through8 included in the basic parameter set, which prediction mode (i) is notincluded in the reduced set RS and (ii) has a smallest index amongprediction modes which are not included in the reduced set RS.

As described above, in the intra prediction, the smaller an indexdesignating a prediction mode is, the more frequently the predictionmode designated by the index tends to be used. Accordingly, in thepresent step, the reduced set derivation section 44 adds, to the reducedset, the prediction mode which tends to be used frequently in the intraprediction.

According to the third example, by carrying out the operations describedabove, it is possible for the reduced set derivation section 44 togenerate the reduced set RS which includes (i) the prediction parameter#43 decoded from each of the plurality of sub blocks belonging to thefirst group and (ii) the prediction parameters, the number of which is2^(n)+1 (n is an integer).

Under a condition that variable-length coding is carried out withrespect to (i) each of the prediction parameters, and simultaneously(ii) the flag indicating whether or not the prediction parameter is thesame as an estimate value, generally, compression efficiency in thevariable-length coding with respect to the prediction parameters tendsto be enhanced in a case where the number of the prediction parametersis 2^(n)+1 (n is an integer), as compared with a case where the numberof the prediction parameters is not 2^(n)+1 (n is an integer).

Accordingly, with the operations described above, the reduced setderivation section 44 can generate the reduced set RS which has highcompression efficiency in the variable-length coding. The video encodingdevice for generating the encoded data #1, including a reduced setderivation section which operates as in the present example, thereforecan generate the encoded data #1 which has high compression efficiency.Further, the video decoding device 1 including the reduced setderivation section 44 which operates as in the present example candecode the encoded data #1 which has high compression efficiency.

Furthermore, in a case where the number of the prediction parameters #43is not 2^(n)+1 (n is an integer), the reduced set derivation section 44can generate the reduced set RS so that the reduced set RS includes thepredetermined prediction parameter. Accordingly, it is possible togenerate the reduced set RS including the prediction mode which tends tobe used frequently.

(Fourth Example of Generation of Reduced Set RS)

In any one of the above examples of generation the respective 3 reducedsets RS shown in (a) through (c) of FIG. 6, the reduced set derivationsection 44 adds, to the reduced set RS, all sorts of predictionparameter #43 which are not identical with each other, among all theprediction parameters #43 decoded from the plurality of sub blocksbelonging to the first group. Note, however, that the present inventionis not limited to this. It is possible to have such an arrangement thatnot all sorts of prediction parameter #43 but only a part of all sortsof prediction parameter #43, among the prediction parameters decodedfrom the plurality of sub blocks belonging to the first group, is addedto the reduced set RS.

Specifically, it is possible to have an arrangement in which, in a groupof parameters decoded from the plurality of sub blocks belonging to thefirst group, only a prediction parameter(s) which has a higherappearance ratio than a predetermined value is added to the reduced setRS. Here, the appearance ratio is defined, for example, as a valueobtained by dividing the number of sub blocks to which the predictionparameter is assigned, among the plurality of sub blocks belonging tothe first group, by the number of all the plurality of sub blocksbelonging to the first group. For example, the number of all theplurality of sub blocks belonging to the first group is Nf, and thenumber of sub blocks, to which a prediction parameter Pa is decoded andassigned, is Npa, among the plurality of sub blocks belonging to thefirst group. In this case, the appearance ratio of the predictionparameter P is defined as Npa/Nf. Further, the appearance ratio can beexpressed by a unit of percentage.

Further, the present example is described below more specifically. Thefollowing description deals with a case where 8 sub blocks (sub blocksSB1 through SB8) belong to the first group.

For example, (i) a prediction mode 0 is decoded with respect to the subblocks SB1, SB2, SB3, and SB4, (ii) a prediction mode 1 is decoded withrespect to sub blocks SB5 and SB6, (iii) a prediction mode 2 is decodedwith respect to the sub block SB7, and (iv) a prediction mode 3 isdecoded with respect to the sub block SB8. In a case where thepredetermined value is set to be 40%, only the prediction mode 0 havingan appearance ratio of 50% is added to the reduced set RS. On the otherhand, in a case where the predetermined value is set to be 20%, theprediction mode 0 having an appearance ratio of 50% and the predictionmode 1 having an appearance ratio of 25% are added to the reduced setRS.

Generally, in a case where the number of sub blocks included in themacro block is large, a large number of prediction parameters areincluded in the reduced set RS. This might make it difficult to reducethe encoding amount effectively.

According to the present example, as to a group of prediction parametersdecoded from the plurality of sub blocks belonging to the first group,the reduced set derivation section 44 adds, to the reduction ser RS,only the prediction parameter(s) which has a higher appearance ratiothan a predetermined value. Accordingly, it is possible to avoid theabove problem that it might become difficult to reduce the encodingamount effectively in a case where the number of sub blocks is large.

As explained in the aforementioned Examples 1 through 4 of generation ofthe reduced set RS, the reduced set RS can be generated on the basis ofthe prediction parameters belonging to the first group. More precisely,the reduced set RS can be generated on the basis of at least one of (i)sorts of prediction parameter, among the plurality of predictionparameters belonging to the first group and (ii) an appearance ratio ofeach of the prediction parameters belonging to the first group.

(Second Prediction Parameter Decoding Section 45)

Next, the following description deals with how the second predictionparameter decoding section 45 operates, with reference to FIG. 7. Thesecond prediction parameter decoding section 45 decodes a predictionparameter P in the encoded data related to each of the plurality of subblocks, included in the sub block encoded data #141 b, which predictionparameter P is used by the group determination section 41 to predicteach of the plurality of sub blocks which are determined as belonging tothe second group.

In other words, by referring to information related to the predictionparameter of each of the plurality of sub blocks belonging to the secondgroup, which information is included in the sub block encoded data #141b, the second prediction parameter decoding section 45 decodes theprediction parameter P used in prediction of each of the plurality ofsub blocks belonging to the second group.

Further, the prediction parameter P thus decoded is outputted as aprediction parameter #45.

FIG. 7 is a flow chart showing an example of how the second predictionparameter decoding section 45 carries out a decoding process.

As shown in FIG. 7, the second prediction parameter decoding section 45first counts the number N of prediction parameters included in thereduced set RS (Step S501).

Next, the second prediction parameter decoding section 45 determineswhether or not the number N of prediction parameters included in thereduced set RS is 1 (Step S502).

In a case where N is 1 (Yes in Step S502), the second predictionparameter decoding section 45 sets, as the prediction parameter P, theonly one prediction parameter included in the reduced set RS (StepS503).

On the other hand, in a case where N is not 1 (No in Step S502), thesecond prediction parameter decoding section 45 derives a predictionparameter estimate value Q (Step S504). Here, the prediction parameterestimate value Q is a prediction parameter used in prediction of a subblock which is located in a position adjacent to the prediction targetsub block on an upper side (or on a left side) with respect to theprediction target sub block. Further, in a case where the sub blocklocated in the position adjacent to the prediction target sub block onthe upper side (or on the left side) has not been decoded, theprediction parameter estimate value Q is a prediction parameter used inprediction of a sub block which (i) has been decoded, (ii) is located onthe upper side (or on the left side) with respect to the predictiontarget sub block, and (iii) is closest to the prediction target subblock among such sub blocks.

Next, the second prediction parameter decoding section 45 decodes a flagindicating whether or not a decoding target prediction parameter and theprediction parameter estimate value Q are identical with each other.Then, the second prediction parameter decoding section 45 substitutes avariable “a” with a value thus decoded.

The following description deals with, as an example, a case where theprediction parameter estimate value Q is identical with one ofprediction parameters included in the reduced set RS. Further, in thefollowing explanation, (i) a variable “a” whose value is 1 means thatthe decoding target prediction parameter is identical with theprediction parameter estimate value Q, and (ii) a variable “a” whosevalue is not 1 means that the decoding target prediction parameter isnot identical with the prediction parameter estimate value Q.

Next, the second prediction parameter decoding section 45 determineswhether or not a value of the variable “a” is 1 (Step S506).

In a case where a value of the variable “a” is 1 (Yes in Step S506), thesecond prediction parameter decoding section 45 sets the predictionparameter estimate value Q as the prediction parameter P (Step S507).

On the other hand, in a case where a value of the variable “a” is not 1(No in Step S506), the second prediction parameter decoding section 45determines whether or not the number N of prediction parameters includedin the reduced set RS is 2 (Step S508).

In a case where N is 2 (Yes in Step S508), the second predictionparameter decoding section 45 sets, as the prediction parameter P, aprediction parameter which (i) is included in the reduced set RS and(ii) is not identical with the prediction parameter estimate value Q(Step S509).

In a case where N is not 2 (No in Step S508), the second predictionparameter decoding section 45 decodes a bit sequence having a length ofceil (log₂ (N−1)) bits, and substitutes a variable “b” with a value thusdecoded (Step S510). Here, the “ceil . . . ” is a ceiling functionhaving a value of a minimum integer among integers not less than a valuein the brackets (this also applies to the following cases). Accordingly,the “ceil . . . ” can be expressed as a function which rounds out avalue in the brackets to obtain an integer, in a case where the value inthe brackets is a positive value.

For example, in a case where N is 5, the second prediction parameterdecoding section 45 decodes the bit sequence having a length of “ceil(log₂ (5-1))=2 bits”, and substitutes the variable “b” with a value thusdecoded. Here, since the length of the bit sequence is 2 bits, a valueof the variable “b” is one of 0, 1, 2, and 3, accordingly.

Next, the second prediction parameter decoding section 45 sets, as theprediction parameter P, a prediction parameter which (i) is included inthe reduced set RS, (ii) is not identical with the prediction parameterestimate value Q, and (iii) has an index which is the (b+1) th smallestindex among such prediction parameters (Step S511).

For example, in a case where a value of the variable “b” is 0, thesecond prediction parameter decoding section 45 sets, as the predictionparameter P, a prediction parameter which (i) is included in the reducedset RS, (ii) is not identical with the prediction parameter estimatevalue Q, and (iii) has the smallest index among such predictionparameters.

Note that, in a case where the prediction parameter estimate value Qderived in the process explained in Step S504 is not identical with anyone of prediction parameters included in the reduced set RS, theprediction parameter estimate value Q is a prediction parameter which(i) is included in the reduced set RS and (ii) has the smallest indexamong the prediction parameters included in the reduced set RS.

An example of how the second prediction parameter decoding section 45carries out a decoding process is as described above. The secondprediction parameter decoding section 45 outputs, as the predictionparameter #45, the prediction parameter P decoded by the aforementionedprocess.

As described above, a video decoding device 1 for decoding encoded datawhich is obtained in such a manner that (i) a difference between anoriginal image and a prediction image is encoded for each of a pluralityof prediction units, and, simultaneously, (ii) selection information isencoded, the selection information indicating which one of a pluralityof prediction parameters, each designating how to generate a predictionimage, is selected for each of the plurality of prediction units,includes: classification means (group determination section 41) forclassifying a plurality of prediction units included in each of aplurality of unit regions constituting the prediction image into a firstgroup and a second group; first selection means (first predictionparameter decoding section 43) for selecting, by referring to firstselection information (information which is included in sub blockencoded data #141 b and is related to a prediction parameter withrespect to each of a plurality of sub blocks belonging to the firstgroup) for each of a plurality of prediction units belonging to thefirst group, among the selection information, a prediction parameterdesignating how to generate a prediction image for each of the pluralityof prediction units belonging to the first group, the first selectionmeans selecting the prediction parameter from a basic set constituted bypredetermined prediction parameters; and second selection means (secondprediction parameter decoding section 45) for selecting, by referring tosecond selection information (information which is included in the subblock encoded data #141 b and is related to a prediction parameter withrespect to each of a plurality of sub blocks belonging to the secondgroup) for each of a plurality of prediction units belonging to thesecond group, among the selection information, a prediction parameterdesignating how to generate a prediction image for each of the pluralityof prediction parameters belonging to the second group, the secondselection means selecting the prediction parameter from a reduced setwhich (i) includes at least a part of the prediction parameter(s)selected by the first selection means (first prediction parameterdecoding section 43) and (ii) is constituted by a predictionparameter(s), the number of which is not more than the number of thepredetermined prediction parameters included in the basic set.

(Another Example of Arrangement of Prediction Parameter Decoding Section144)

In the above explanation, the prediction parameter decoding section 144has an arrangement in which the reduced set derivation section 44generates a reduced set RS per macro block. Note, however, that thepresent invention is not limited to the arrangement.

For example, the prediction parameter decoding section 144 can have anarrangement in which the reduced set derivation section 44 generates areduced set RS per sub block, and the second prediction parameterdecoding section decodes a prediction parameter with respect to aprediction target sub block, on the basis of the reduced set RSgenerated per sub block.

With the arrangement, the reduced set derivation section 44 can generatea reduced set RS by carrying out the following process, as shown in (a)of FIG. 8.

(Step S701)

First, the reduced set derivation section 44 sets the reduced set RS tobe empty, so as to initialize the reduced set RS.

(Step S702)

Next, the reduced set derivation section 44 sets, as a proximity subblock region NSR, a region constituted by sub blocks in the vicinity ofthe prediction target sub block.

(b) of FIG. 8 is a view showing an example of the proximity sub blockregion NSR. As shown in (b) of FIG. 8, for example, the proximity subblock NSR is constituted by sub blocks in the vicinity of the predictiontarget sub block, and a distance between the prediction target sub blockand the sub blocks in the vicinity of the prediction target sub block isdefined as being within 3 sub blocks in terms of a city block distance(unit: sub block). Here, the city block distance is defined as a sum ofabsolute differences, each of which corresponds to an absolutedifference between two coordinates along each direction.

Further, as shown in (b) of FIG. 8, the proximity sub block region NSRcan contain sub blocks belonging to a macro block other than the macroblock to which the prediction target sub block belongs to.

(Step S703)

Next, the reduced set derivation section 44 adds, to the reduced set RS,a prediction parameter(s) which has been decoded, among predictionparameters with respect to sub blocks included in the proximity subblock region NSR.

Note that, in a case where one prediction parameter is provided for aplurality of sub blocks, among the sub blocks included in the proximitysub block region NSR, the reduced set derivation section 44 adds, forsuch a plurality of sub blocks, only the one prediction parameter to thereduced set RS.

With the operations described above, the reduced set derivation section44 can generate the reduced set RS per macro block. Further, the secondprediction parameter decoding section 45 can decode the predictionparameter with respect to the prediction target sub block, on the basisof the reduced set RS generated per sub block.

Generally, a prediction parameter with respect to the prediction targetsub block has a correlation with a prediction parameter with respect toanother sub block located in the vicinity of the prediction target subblock. Accordingly, the prediction parameter(s) included in the reducedset RS generated by the above process is highly likely to include a mostappropriate prediction parameter in prediction of a sub block belongingto the second group. Further, the number of prediction parametersincluded in the reduced set RS generated by the above process isgenerally smaller than the number of prediction parameters which can beselected for the first group.

Accordingly, the video encoding device for generating the encoded data#1, having an arrangement corresponding to the arrangement describedabove, can generate, without sacrificing encoding efficiency, theencoded data #1 whose encoding amount is small. Furthermore, the videodecoding device 1 having the arrangement described above can decode theencoded data #1 thus generated, whose encoding amount is small.

Note that, in a case where, for the plurality of sub blocks included inthe proximity sub block region NSR, there is no prediction parameterwhich has been decoded, the second prediction parameter decoding section45 selects a prediction parameter from the basic parameter set, forexample.

Moreover, the reduced set 44 of the present example can have anarrangement in which the reduced set RS is derived in substantially thesame manner as the process explained in any one of “Example 1 ofgeneration of reduced set RS” to the “Example 4 of generation of reducedset RS”. Note, however, that, in that case, the “first group” in each ofthe “Example 1 of generation of reduced set RS” to the “Example 4 ofgeneration of reduced set RS” corresponds to the “proximity sub blockregion NSR” in the present example.

Further, in the above explanation, the reduced set RS is used withrespect to the second group. Note, however, that the present inventionis not limited to this. The aforementioned process can be applied to allthe sub blocks in the macro block. That is, it is possible to have anarrangement in which, with respect to each of the sub blocks in themacro block, a prediction parameter is decoded on the basis of thereduced set RS generated per sub block.

The video encoding device for generating the encoded data #1, having anarrangement corresponding to the arrangement described above, canfurther reduce the encoding amount of the prediction parameters withrespect to the sub blocks in the macro block. Accordingly, the videoencoding device can generate the encoded data #1 whose encoding amountis further reduced. Moreover, the video decoding device 1 having thearrangement described above decodes the encoded data #1 thus generated.

(Prediction Image Generation Section 145)

The following description deals with how the prediction image generationsection 145 generates a prediction image #145.

The prediction image generation section 145 generates, in accordancewith a prediction direction (prediction mode) indicated by theprediction parameter #144, a prediction pixel value of each pixel(prediction target pixel) of the prediction image #145 (predictiontarget sub block) in the following manner, for example. Note that, thefollowing explanation deals with an example where the predictionparameter #144 is one of the prediction modes 0 through 8 shown in FIG.5.

The prediction image generation section 145 assigns a prediction modeindicated by the prediction parameter #144 to the prediction targetpixel, and then, carries out the following operation.

-   -   In a case where the prediction mode thus assigned is not the        prediction mode 2 (DC prediction), the prediction image        generation section 145 sets, as a pixel value of the prediction        target pixel, a pixel value of a pixel (hereinafter, referred to        as “closest pixel”) closest to the prediction target pixel,        among pixels which (i) are located on a virtual segment        extending from a position of the prediction target pixel in a        direction opposite to a prediction direction and (ii) have been        decoded. Further, it is possible to set, as the pixel value of        the prediction target pixel, a value calculated by use of (i)        the pixel value of the closest pixel and (ii) a pixel value(s)        of a pixel(s) in the vicinity of the closest pixel.    -   In a case where (i) the prediction mode thus assigned is the        prediction mode 2, and (ii) a sub block adjacent to the        prediction target sub block on an upper side with respect to the        prediction target sub block (hereinafter, referred to as “upper        sub block”), and a sub block adjacent to the prediction target        sub block on a left side with respect to the prediction target        sub block (hereinafter, referred to as “left sub block”) have        been decoded, an average value of (a) a pixel value of a pixel        located in a lowest sequence of the upper sub block and (b) a        pixel value of a pixel in a rightmost sequence of the left sub        block is used as the pixel value of the prediction target pixel.    -   In a case where (i) the prediction mode thus assigned is the        prediction mode 2, (ii) the upper sub block has been decoded,        and (iii) the left sub block has not been decoded, an average        value of (a) the pixel value of the pixel in the lowest sequence        of the upper sub block and (b) a pixel value of a pixel in a        rightmost sequence in a sub block which is closest to the        prediction target sub block on the left side with respect to the        prediction target sub block (hereinafter, referred to as “left        closest sub block”), is used as the pixel value of the        prediction target pixel.    -   In a case where (i) the prediction mode thus assigned is the        prediction mode 2, (ii) the upper sub block has not been        decoded, and (iii) the left sub block has been decoded, an        average value of (a) a pixel value of a pixel in a lowest        sequence of a sub block which is closest to the prediction        target sub block on the upper side with respect to the        prediction target sub block (hereinafter, referred to as “upper        closest sub block”), and (b) the pixel value of the pixel in the        rightmost sequence of the left sub block is used as the pixel        value of the prediction target pixel.    -   In a case where (i) the prediction mode thus assigned is the        prediction mode 2, and (ii) neither the upper sub block nor the        left sub block has been decoded, an average value of (a) the        pixel value of the pixel in the lowest sequence of the upper        closest sub block and (b) the pixel value of the pixel in the        rightmost of the left closest sub block is used as the pixel        value of the prediction target pixel.

An example of how the prediction image generation section 145 generatesthe prediction image #145 is described below more specifically, withreference to FIG. 9. The following description deals with a case wherethe prediction target sub block is constituted by 4×4 pixels.

FIG. 9 is a view showing (i) each pixel (prediction target pixel) of theprediction target sub block constituted by 4×4 pixels, and (ii) pixels(reference pixels) in the vicinity of the prediction target sub block.As shown in FIG. 9, the prediction target pixels are provided with signs“a” through “p”, respectively, and the reference pixels are providedwith signs “A” through “M”, respectively. A pixel value of a pixel “X”(“X” is one of “a” through “p”, or one of “A” through “M”) isrepresented by a sign “X”. Further, all the reference pixels “A” through“M” have been decoded.

(Prediction Mode 0)

In a case where the prediction mode thus assigned is the prediction mode0, the prediction image generation section 145 generates the pixelvalues “a” through “p” by use of the following formulas.a,e,i,m=A,b,f,j,n=B,c,g,k,o=C,d,h,l,p=D(Prediction Mode 2)

In a case where the prediction mode thus assigned is the prediction mode2 (DC prediction), the prediction image generation section 145 generatesthe pixel values “a” through “p” by use of the following formula.a˜p=ave(A,B,C,D,I,J,K,L)Here, the “ave ( . . . )” means to obtain an average of elementsincluded in the brackets.(Prediction Mode 4)

In a case where the prediction mode thus assigned is the prediction mode4, the prediction image generation section 145 generates the pixelvalues “a” through “p” by use of the following formula.d=(B+(C×2)+D+2)>>2,c,h=(A+(B×2)+C+2)>>2,b,g,l=(M+(A×2)+B+2)>>2,a,f,k,p=(I+(M×2)+A+2)>>2,e,j,o=(J+(I×2)+M+2)>>2,i,n=(K+(J×2)+I+2)>>2,m=(L+(K×2)+J+2)>>2Here, “>>” represents a right shift operation. A value of “x>>s” (x ands are arbitral integers) is equal to a value obtained in such a mannerthat a fractional part of a value of “x/(2^s) is rounded down.

Further, for a prediction mode other than the prediction modes describedabove, the prediction image generation section 145 can calculate thepixel values “a” through “p” in the same manner as described above.

<Additional Matters Related to Video Encoding Device>

The video encoding device of the present invention is thus describedabove. Note, however, that the present invention is not limited to thearrangements described above.

(Additional Matter 1)

For example, the second prediction parameter decoding section 45 candetermine, in accordance with a flag included in the encoded data #1,whether or not the reduced set RS is used in decoding the predictionparameter #45.

More specifically, for example, in a case where a value of the flagincluded in the encoded data #1 is 1, the second prediction parameterdecoding section 45 decodes the prediction parameter #45 by use of thereduced set RS. On the other hand, in a case where a value of the flagincluded in the encoded data #1 is 0, the second prediction parameterdecoding section 45 decodes the prediction parameter #45 by use of thebasic parameter set in place of the reduced set RS, for example.

With the arrangement, it is possible to reduce an amount of a processfor decoding the prediction parameter #45.

(Additional Matter 2)

Further, the second prediction parameter decoding section 45 candetermine, in accordance with the number of sub blocks included in themacro block, whether or not the reduced set RS is used in decoding theprediction parameter #45.

More specifically, in a case where the number of sub blocks included inthe macro block is not less than 16, the second prediction parameterdecoding section 45 decodes the prediction parameter #45 by use of thereduced set RS. On the other hand, in a case where the number of subblocks included in the macro block is less than 16, the secondprediction parameter decoding section 45 decodes the predictionparameter #45 by use of the basic parameter set in place of the reducedset RS, for example.

With the arrangement, it is possible to reduce an amount of the processfor decoding the prediction parameter #45.

(Additional Matter 3)

Further, in the above explanations, the basic parameter set is a set ofprediction parameters, as shown in FIG. 5. However, the presentinvention is not limited to this.

For example, it is possible to have an arrangement in which theprediction parameter decoding section 144 employs a parameter set whosemain direction is a horizontal direction, as shown in (a) of FIG. 16, ora parameter set whose main direction is a vertical direction, as shownin (b) of FIG. 16.

More specifically, for example, in a case where there is an edge in thehorizontal direction in the macro block, the prediction parameterdecoding section 144 uses, as the basic parameter set, the parameter setwhose main direction is the horizontal direction, as shown in (a) ofFIG. 16. Meanwhile, in a case where there is an edge in the verticaldirection in the macro block, the prediction parameter decoding section144 uses, as the basic parameter set, the parameter set whose maindirection is the vertical direction, as shown in (b) of FIG. 16.

Further, in a case where such a plurality of basic parameter sets areselectively used, and the parameter set shown in (a) of FIG. 16 or theparameter set shown in (b) of FIG. 16 is selected as the basic parameterset, the second prediction parameter decoding section 45 decodes theprediction parameter #45 by use of the reduced set RS. In a case wherethe parameter set shown in FIG. 5 is selected as the basic parameterset, the second prediction parameter decoding section 45 decodes theprediction parameter #45 by use of the parameter set shown in FIG. 5 asthe basic parameter set.

With the arrangement, it is possible to use the reduced set RS inaccordance with a characteristic of an image in the macro block.

(Video Encoding Device)

The following description deals with a video encoding device (imageencoding device) 2 of the present embodiment with reference to FIGS. 10through 14. FIG. 10 is a block diagram illustrating an arrangement ofthe video encoding device 2. The video encoding device 2 includes aheader information determination section 21, a header informationencoding section 22, an MB setting section 23, an MB encoding section24, a variable-length code multiplexing section 25, an MB decodingsection 26, and a frame memory 27 (see FIG. 10).

To put it shortly, the video encoding device 2 generates encoded data #1by encoding an input image #100, and outputs the encoded data #1 thusgenerated.

The header information determination section 21 determines headerinformation on the basis of the input image #100. The header informationthus determined is outputted as header information #21. The headerinformation #21 includes an image size of the input image #100. Theheader information #21 is supplied to the MB setting section 23, and isalso supplied to the header information encoding section 22.

The header information encoding section 22 encodes the headerinformation #21 and outputs the header information #21 thus encoded asencoded header information #22. The encoded header information #22 issupplied to the variable-length code multiplexing section 25.

On the basis of the header information #21, the MB setting section 23divides the input image #100 into a plurality of macro blocks, andoutputs a plurality of macro block images #23 related to the respectiveplurality of macro blocks. The plurality of macro block images #23 aresupplied to the MB encoding section 24 sequentially.

The MB encoding section 24 encodes the plurality of macro block images#23 which are sequentially supplied, so as to generate MB encoded data#24. The MB encoded data #24 thus generated is supplied to thevariable-length code multiplexing section 25. An arrangement of the MBencoding section 24 will be described later, and therefore anexplanation of the arrangement of the MB encoding section 24 is omittedhere.

The variable-length code multiplexing section 25 multiplexes the encodedheader information #22 and the MB encoded data #24, so as to generatethe encoded data #1. Then, the variable-length code multiplexing section25 outputs the encoded data #1.

The MB decoding section 26 sequentially decodes the MB encoded data #24corresponding to each of the plurality of macro blocks thus supplied, soas to generate a decoded image #26 corresponding to each of theplurality of macro blocks. Then, the MB decoding section 26 outputs thedecoded image #26. The decoded image #26 is supplied to the frame memory27.

The decoded image #26 thus supplied is stored in the frame memory 27.When a specific macro block is encoded, decoded images corresponding to,respectively, all macro blocks provided before the macro block in araster scan order have been stored in the frame memory.

(MB Encoding Section 24)

The following description deals with the MB encoding section 24 withreference to another drawing more specifically.

FIG. 11 is a block diagram illustrating an arrangement of the MBencoding section 24. The MB encoding section 24 includes a sub blockdividing section 241, a prediction parameter determination section 242,a prediction parameter encoding section 243, a prediction residualgeneration section 244, a transform coefficient encoding section 245, aprediction residual decoding section 246, a sub block decoded imagegeneration section 247, a prediction image generation section 248, andan MB encoded data generation section 249 (see FIG. 11).

The sub block dividing section 241 divides the macro block image #23into a plurality of sub blocks. The sub block dividing section 241sequentially outputs, in a predetermined order, (i) sub block positioninformation #241 a indicating a position of each of the plurality of subblocks in the macro block, which plurality of sub blocks constitute themacro block, and (ii) a sub block image #241 b which is image datarelated to the sub block indicated by the sub block position information#241 a.

Note that it is preferable that the sub block dividing section 241 (i)outputs sub block position information #241 a related to each of aplurality of sub blocks belonging to a first group (described later) anda sub block image #241 b related to each of the plurality of the subblocks belonging to the first group, and then (ii) outputs sub blockposition information #241 a related to each of a plurality of sub blocksbelonging to a second group (described later) and a sub block image #241b related to each of the plurality of sub blocks belonging to the secondgroup. For example, it is preferable to have an arrangement in which theplurality of sub blocks belonging to the first group are first scannedin the raster scan order, and then, the plurality of sub blocksbelonging to the second group are scanned in the raster scan order.

The prediction parameter determination section 242 determines aprediction parameter #242 used to generate a prediction image related tothe sub block indicated by the sub block position information #241 a,and outputs the prediction parameter #242 thus determined. Further, theprediction parameter encoding section 243 encodes the predictionparameter #242, so as to obtain an encoded prediction parameter #243.Then, the prediction parameter encoding section 243 outputs the encodedprediction parameter #243. An arrangement of the prediction parameterdetermination section 242 and an arrangement of the prediction parameterencoding section 243 will be described later, and therefore explanationsof these arrangements are omitted here.

On the basis of the sub block position information #241 a, theprediction residual generation section 244 (i) specifies the sub blockwhich is to be a prediction target, and (ii) generates a predictionresidual #244 which is a difference image between the sub block image#241 b corresponding to the sub block and a prediction image #248generated by the prediction image generation section 248.

The transform coefficient encoding section 245 applies, with respect tothe prediction residual #244, frequency transform whose size isidentical with that of the sub block, so as to generate a transformcoefficient of the prediction residual #244.

Further, the transform coefficient encoding section 245 generates aquantization transform coefficient #245 a by quantizing the transformcoefficient, and then generates a variable-length code by applying avariable-length code method (such as CABAC and CAVLC) with respect tothe quantization transform coefficient #245 a. After that, the transformcoefficient encoding section 245 outputs the variable-length code asencoded data #245 b.

The prediction residual decoding section 246 carries out inversequantization with respect to the quantization transform coefficient #245a, and then, generates a decoded residual #246 by applying inversetransform of the frequency transform (inverse frequency transform) withrespect to the quantization transform coefficient #245 a. After that,the prediction residual decoding section 246 outputs the decodedresidual #246.

Note that the present invention is not limited to the processes carriedout by the prediction residual generation section 244, the transformcoefficient encoding section 245, and the prediction residual decodingsection 246. For example, the transform coefficient encoding section 245can omit the frequency transform, and can directly quantize theprediction residual.

The sub block decoded image generation section 247 generates a sub blockdecoded image #247 by adding the prediction image #248 and the decodedresidual #246 to each other, and outputs the sub block decoded image#247.

The prediction image generation section 248 generates a prediction image#248 corresponding to the prediction target sub block, on the basis ofthe prediction parameter #242, a decoded image #27, and the sub blockdecoded image #247, and outputs the prediction image #248 thusgenerated. A method identical with the method of generating theprediction image #145 by the prediction image generation section 145described above can be applied to a specific method of generating theprediction image #248 by the prediction image generation section 248.

The MB encoded data generation section 249 accumulates (i) the encodeddata #245 b related to each of the plurality of sub blocks and (ii) theencoded prediction parameter #243 related to each of the plurality ofsub blocks, so as to cause these to be integrated with each other permacro block. The MB encoded data generation section 249 thus generatesMB encoded data #24 which is encoded data whose unit is a macro block,and then, outputs the MB encoded data #24.

The following description deals with the prediction parameterdetermination section 242 and the prediction parameter encoding section243 with reference to another drawing.

(Prediction Parameter Determination Section 242)

FIG. 12 is a block diagram illustrating an arrangement of the predictionparameter determination section 242. The prediction parameterdetermination section 242 includes a group determination section 51, aswitch section 52, a first prediction parameter determination section53, a reduced set derivation section 54, and a second predictionparameter determination section 55 (See FIG. 12).

The group determination section 51 classifies the sub block indicated bythe sub block position information #241 a into one of a plurality ofgroups, and outputs, to the switch section 52, group information #51indicating a result of the classification.

The group determination section 51 can classify the plurality of subblocks into the first group and the second group, as explained abovewith reference to (a) through (f) of FIG. 4, for example.

Further, the group determination section 51 can classify the pluralityof sub blocks into a plurality of groups by use of differentclassification method for the plurality of macro blocks. For example,the group determination section 51 can classify a plurality of subblocks constituting a macro block MB1 into two groups as shown in (a)and (b) of FIG. 4, while classifying a plurality of sub blocksconstituting a macro block MB2, which is different from the macro blockMB1, into two groups as shown in (c) and (d) of FIG. 4. With thearrangement in which different classification methods are used for theplurality of macro blocks as described above, it is preferable that thegroup determination section 51 outputs a flag indicating whichclassification method has been used. By transmitting the flag to thevideo decoding device for decoding the encoded data #1, the videodecoding device can identify which one of classification methods hasbeen used by the group determination section 51.

On the basis of the group information #51, the switch section 52transmits, to one of the first prediction parameter determinationsection 53 and the second prediction parameter determination section 55,the sub block encoded data #241 b which is encoded data related to thesub block indicated by the sub block position information #241 a.

Specifically, in a case where the group determination section 51classifies the sub block indicated by the sub block position information#241 a into the first group, the switch section 52 transmits the subblock encoded data #241 b to the first prediction parameterdetermination section 53. On the other hand, in a case where the groupdetermination section 51 classifies the sub block indicated by the subblock position information #241 a into the second group, the switchsection 52 transmits the sub block encoded data #241 b to the secondprediction parameter determination section 45.

On the basis of the decoded image #27, the sub block decoded image #247,and the sub block encoded data #241 b, the first prediction parameterdetermination section 53 determines (selects) a prediction parameter #53used to generate a prediction image related to each of the plurality ofsub blocks belonging to the first group. Then, the first predictionparameter determination section 53 outputs the prediction parameter #53.Further, the prediction parameter #53 is also supplied to the reducedset derivation section 54.

In a case where, for example, the prediction parameter is an intraprediction mode defined in the H.264/MPEG-4 AVC standard, the firstprediction parameter determination section 53 (i) selects, with respectto each of the plurality of sub blocks belonging to the first group, oneof prediction modes in the basic parameter set which has been describedabove with reference to FIG. 5, and (ii) outputs the prediction modethus selected.

Note that the present invention is not limited to the specific method ofdetermining the specific prediction parameter #53, carried out by thefirst prediction parameter determination section 53. For example, thefirst prediction parameter determination section 53 can determine theprediction parameter #53 with respect to each of the plurality of subblocks belonging to the first group so that a difference between theprediction image of the sub block and the input image #100 becomessmallest. For example, in a case where, with respect to a sub block SB1belonging to the first group, a difference between a prediction imagegenerated by use of the prediction mode 1 in the basic parameter set andthe input image #100 becomes smallest, as compared with a case whereanother prediction mode in the basic parameter set is used, the firstprediction parameter determination section 53 selects the predictionmode 1 with respect to the sub block SB1. In a case where, with respectto a sub block SB2 belonging to the first group, a difference between aprediction image generated by use of the prediction mode 2 in the basicparameter set and the input image #100 becomes smallest, as comparedwith a case where another prediction mode in the basic parameter set isused, the first prediction parameter determination section 53 selectsthe prediction mode 2 with respect to the sub block SB2.

(a) of FIG. 13 is a view showing an example of a prediction mode whichis selected by the first prediction parameter determination section 53with respect to each of the plurality of sub blocks belonging to thefirst group, among the plurality of sub blocks constituting the macroblock MB. According to the example shown in (a) of FIG. 13, the firstprediction parameter determination section 53 selects, with respect toeach of the plurality of sub blocks belonging to the first group, one ofthe prediction mode 1, the prediction mode 6, and the prediction mode 8.In the present example, each of the prediction modes 1, 6, and 8 issupplied to the reduced set derivation section 54 as the predictionparameter #53.

With the operations described above, the reduced set derivation section54 is supplied with the prediction parameter #53 related to each of theplurality of sub blocks belonging to the first group.

An arrangement of the reduced set derivation section 54 is the same asthat of the reduced set derivation section 44 described above. That is,the reduced set derivation section 54 generates the reduced set RS byuse of the prediction parameter #53. A method of generating the reducedset RS by the reduced set derivation section 54 is the same as themethod of generating the reduced set RS by the reduced set derivationsection 44 described above.

Note that, in a case where a plurality of generation methods areemployed by the reduced set derivation section 54 selectively, it ispreferable that the reduced set derivation section 54 outputs a flagindicating which one of the plurality of generation methods has beenselected. By transmitting the flag to the video decoding device fordecoding the encoded data #1, it becomes possible for the video decodingdevice to identify which one of the plurality of generation methods hasbeen employed by the reduced set derivation section 54.

(b) of FIG. 13 is a view showing an example of the reduced set RS, whichis generated by the reduced set derivation section 54 in a case whereeach of the prediction modes shown in (a) of FIG. 13 is supplied as theprediction parameter #53. According to the present example, the reducedset RS is constituted by a prediction mode 1, a prediction mode 8, and aprediction mode 6 (see (b) of FIG. 13).

The second prediction parameter determination section 55 selects, fromamong prediction parameters included in the reduced set RS, a predictionparameter #55 used to generate a prediction image related to each of theplurality of sub blocks belonging to the second group. Then, the secondprediction parameter determination section 55 outputs the predictionparameter #55.

The present invention is not limited to a specific method of determiningthe prediction parameter #55 by the second prediction parameterdetermination section 55. For example, the second prediction parameterdetermination section 55 selects, from among the prediction parametersincluded in the reduced set RS with respect to each of the plurality ofsub blocks belonging to the second group, such a prediction parameter#55 that a prediction image of the sub block can be generated in themost appropriate manner.

(c) of FIG. 13 is a view showing an example of a prediction mode, whichis selected from among the prediction modes included in the reduced setRS shown in (b) of FIG. 13 by the second prediction parameterdetermination section 55 with respect to each of the plurality of subblocks belonging to the second group, among the plurality of sub blocksconstituting the macro block MB. As shown in (c) of FIG. 13, one of theprediction modes 1, 6, and 8, included in the reduced set RS shown in(b) of FIG. 13, is selected with respect to each of the plurality of subblocks belonging to the second block.

Generally, optimum prediction parameters with respect to a plurality ofsub blocks constituting a macro block have a correlation with eachother. Accordingly, a prediction parameter selected with respect to eachof the plurality of sub blocks belonging to the first group is highlylikely to be an optimum prediction parameter with respect to each of theplurality of sub blocks belonging to the second group.

Further, as described above, the second prediction parameterdetermination section 55 selects, from among the prediction parametersincluded in the reduced set RS, the prediction parameter #55 withrespect to each of the plurality of sub blocks belonging to the secondgroup. Accordingly, it is possible to reduce an encoding amount of theprediction parameters #55, as compared with a case where the reduced setRS is not used.

For example, as described later, in a case where a prediction mode and aflag indicating whether or not the prediction mode is identical with anestimate value are simultaneously encoded, the three prediction modesincluded in the reduced set RS shown in (b) of FIG. 13 can be encodedwith an encoding amount of ceil (log₂ (3−1))=1 bit. Meanwhile, in a casewhere the second prediction parameter determination section 55 selects aprediction parameter from the basic parameter set constituted by 9prediction modes, without using the reduced set RS, a necessary encodingamount becomes ceil (log₂ (9−1))=3 bits. Furthermore, in the aboveexample, the second group is constituted by 8 sub blocks. Accordingly,as compared with the case where the reduced set RS is not used, it ispossible to reduce, per macro block, the encoding amount by 3×8−1×8=16bits, by using the reduced set RS.

Generally, in a case where a prediction parameter and the flagindicating whether or not the prediction parameter is identical with theestimate value are simultaneously encoded, it is possible to reduce, permacro block, the encoding amount by Ngs×(ceil (log₂ (Nfs−1))−ceil (log₂(Nrs−1))) bits (where: Nfs is the number of prediction parameters whichcan be selected with respect to each of the plurality of sub blocksbelonging to the first group; Nrs is the number of prediction parametersincluded in the reduced set RS; and Ngs is the number of sub blocksincluded in the second group) by use of the reduced set RS.

As described above, by using the reduced set RS, it is possible toreduce the encoding amount necessary to encode a prediction parameter,without sacrificing encoding efficiency.

(Another Example of Arrangement of Prediction Parameter DeterminationSection 242)

In the above explanation as to the prediction parameter determinationsection 242, the reduced set derivation section 54 generates the reducedset RS per macro block. Note, however, that the present invention is notlimited to the arrangement.

That is, the prediction parameter determination section 242 can havesuch an arrangement that (i) the reduced set derivation section 54generates the reduced set RS per sub block, and (ii) the secondprediction parameter determination section 55 determines a predictionparameter with respect to a prediction target sub block on the basis ofthe reduced set RS generated per sub block.

With the arrangement, the reduced set derivation section 54 carries outthe same operations as those of the reduced set derivation section 44,explained in the above (Step S701) through (Step S703) of (Anotherexample of arrangement of reduction parameter decoding section 144)described above. Note, however, that the decoded prediction parameter inthe above (Step S701) through (Step S703) of (Another example ofarrangement of prediction parameter decoding section 144) corresponds tothe encoded prediction parameter in the present example.

With the arrangement, the reduced set derivation section 54 can generatethe reduced set RS per sub block. Further, the second predictionparameter determination section 55 can determine the predictionparameter with respect to the prediction target sub block, on the basisof the reduced set RS generated per sub block.

Generally, a prediction parameter with respect to the prediction targetparameter has a correlation with a prediction parameter with respect toanother sub block located in the vicinity of the prediction target subblock. Accordingly, a prediction parameter included in the reduced setRS generated by the operations described above is highly likely to be anoptimum prediction parameter in prediction of a sub block belonging tothe second group. Further, the number of prediction parameters includedin the reduced set RS generated by the operations described above isgenerally smaller than the number of prediction parameters which can beselected with respect to sub blocks included in the first group.

Accordingly, the video decoding device 1 having the arrangementdescribed above can generate encoded data #1 whose encoding amount issmall, without sacrificing encoding efficiency.

Note that, in a case where there is no encoded prediction parameter withrespect to a plurality of sub blocks included in the proximity sub blockregion NSR, the second prediction parameter determination section 55selects a prediction parameter from the basic parameter set, forexample.

Furthermore, the reduction set derivation section 54 of the presentexample can derive the reduced set RS by carrying out operationssubstantially similar to the operations described in the above (Example1 of generation of reduced set RS) through (Example 4 of generation ofreduced set RS). In this case, however, the “first group” described inthe above (Example 1 of generation of reduced set RS) through (Example 4of generation of reduced set RS) corresponds to the “proximity sub blockregion NSR” in the present example.

Moreover, in the above explanation, the reduced set RS is used withrespect to the second group. Note, however, that the present inventionis not limited to this. It is possible to apply the aforementionedoperations to all the sub blocks in the macro block. That is, it ispossible to have an arrangement in which, on the basis of a reduced setRS generated per sub block, a prediction parameter is determined withrespect to each of the sub blocks in the macro block.

With the arrangement, it is possible to reduce an encoding amount ofprediction parameters with respect to the sub blocks in the macro block.Accordingly, the video encoding device 1 having the arrangementdescribed above can generate encoded data #1 whose encoding amount isfurther reduced.

(Prediction Parameter Encoding Section 243)

Next, the following description deals with the prediction parameterencoding section 243 with reference to FIG. 14. FIG. 14 is a blockdiagram illustrating an arrangement of the prediction parameter encodingsection 243. The prediction parameter encoding section 243 includes agroup determination section 61, a switch section 62, a first predictionparameter encoding section 63, a reduced set derivation section 64, asecond prediction parameter encoding section 65 (see FIG. 14).

The group determination section 61 has an arrangement which issubstantially similar to that of the group determination section 51described above. That is, the group determination section 61 (i)classifies a sub block indicated by sub block position information #241a into one of a plurality of predetermined groups, and (ii) outputs, tothe switch section 62, group information #61 indicating a result of theclassification. Note that, the group determination section 61 employsthe same classification method as the classification method used by thegroup determination section 51, so as to classify each sub block into acorresponding one of the plurality of predetermined groups.

On the basis of the group information #61, the switch section 62transmits a prediction parameter #242 related to a sub block indicatedby the sub block position information #241 a to one of the firstprediction parameter encoding section 63 and the second predictionparameter encoding section 65.

Specifically, in a case where the group determination section 61classifies the sub block indicated by the sub block position information#241 a into the first group, the switching section 62 transmits theprediction parameter #242 to both the first prediction parameterencoding section 63 and the reduced set derivation section 64. On theother hand, in a case where the group determination section 61classifies the sub block indicated by the sub block position information#241 a into the second group, the switching section 62 transmits theprediction parameter #242 to the second prediction parameter encodingsection 65.

The first prediction parameter encoding section 63 encodes theprediction parameter #242 related to each of the plurality of sub blocksbelonging to the first group, so as to generate an encoded predictionparameter #63. Then, the first prediction parameter encoding section 63outputs the encoded prediction parameter #63.

Specifically, the first prediction parameter encoding section 63 firstsets, as an estimate value with respect to each of the plurality of subblocks belonging to the first group, a prediction parameter selectedwith respect to another sub block located in the vicinity of the subblock.

Next, the first prediction parameter encoding section 63 encodes a flagindicating whether or not the prediction parameter selected with respectto the sub block is different from the estimate value. Further, in acase where the prediction parameter selected with respect to the subblock is different from the estimate value, the first predictionparameter encoding section 63 encodes the prediction parameter.

Here, in a case where the prediction parameter with respect to each ofthe plurality of sub blocks belonging to the first group is selectedfrom the basic parameter set, the prediction parameter can be expressedby a code of 1 bit or a code of 4 bits (including the flag).

As described above, by carrying out encoding by use of the estimatevalue, it is possible to enhance compressibility in encoding theprediction parameter #242 related to each of the plurality of sub blocksbelonging to the first group.

Note that the first prediction parameter encoding section 63 can encodethe prediction parameter #242 itself, related to each of the pluralityof sub blocks belonging to the first group.

The reduced set derivation section 64 generates the reduced set RS byuse of the prediction parameter #242 related to each of the plurality ofsub blocks belonging to the first group. A method of generating thereduced set RS by the reduced set derivation section 64 is the same asthe method of generating the reduced set RS by the reduced setderivation section 44 described above, and therefore explanation of themethod of generating the reduced set RS is omitted here.

The second prediction parameter encoding section 65 encodes a predictionparameter which is included in the reduced set RS, that is, theprediction parameter selected with respect to each of the plurality ofsub blocks belonging to the second group, so as to generate an encodedprediction parameter #65. Then, the second prediction parameter encodingsection 65 outputs the encoded prediction parameter #65.

Specifically, the second prediction parameter encoding section 65 firstsets, as an estimate value with respect to each of the plurality of subblocks belonging to the second block, a prediction parameter selectedwith respect to another sub block located in the vicinity of the subblock.

In a case where the number Nrs of prediction parameters included in thereduced set RS is 1, the second prediction parameter encoding section 65does not encode any prediction parameter, and finishes an encodingprocess of a second prediction parameter.

Next, the second prediction parameter encoding section 65 encodes a flagindicating whether or not the prediction parameter selected with respectto the sub block is different from the estimate value. Further, in acase where the prediction parameter selected with respect to the subblock is different from the estimate value, the second predictionparameter encoding section 65 encodes the prediction parameter.

Here, in a case where Nrs is 2, the second prediction parameter encodingsection 65 finishes the encoding process of the second predictionparameter.

In a case where Nrs is not 2, the prediction parameter included in thereductions set can be expressed by a code of ceil (log₂ (Nrs−1)) bits.

Further, the number Nrs of prediction parameters included in the reducedset RS is generally smaller than the number of prediction parameterswhich can be selected with respect to the plurality of sub blocksbelonging to the first group.

Accordingly, by employing the reduced set RS, it is possible to encode aprediction parameter selected with respect to each of the plurality ofsub blocks belonging to the second group, while having a reduction inencoding amount.

Further, as described above, by carrying out encoding by use of anestimate value, it is possible to enhance compressibility in encodingthe prediction parameter #242 related to each of the plurality of subblocks belonging to the second group.

Note that it is possible to have an arrangement in which the secondprediction parameter encoding section 65 encodes the predictionparameter itself, selected with respect to each of the plurality of subblocks belonging to the second group.

<Additional Matters Related to Video Encoding Device>

The video encoding device 2 of the present invention is thus explainedabove. Note, however, that the present invention is not limited to thearrangements described above.

(Additional Matter 1′)

For example, it is possible to have an arrangement in which the secondprediction parameter determination section (i) finds a spatialcorrelation between prediction parameters in the macro block, and (ii),in a case where the spatial correlation thus found is small, determinesthe prediction parameter #55 without using the reduced set RS. Further,it is preferable to have an arrangement in which, in a case where thesecond prediction parameter determination section 55 determines theprediction parameter #55 without using the reduced set RS, the videoencoding device 2 (i) encodes a flag indicating that the reduced set RSis not used, and (ii) transmits the flag thus encoded to the videodecoding device.

With the arrangement, it is possible to determine the predictionparameter #55 without using the reduced set RS, in a case where thespatial correlation between the prediction parameters is small. It istherefore possible to suppress an amount of a process for determining aprediction parameter.

(Additional Matter 2′)

Further, it is possible to have an arrangement in which the secondprediction parameter determination section 55 determines, in accordancewith the number of sub blocks included in the macro block, whether ornot the reduced set RS is used to determine the prediction parameter#55.

More specifically, in a case where the number of sub blocks included inthe macro block is not less than 16, the second prediction parameterdetermination section 55 determines the prediction parameter #55 by useof the reduced set RS. On the other hand, in a case where the number ofsub blocks included in the macro block is less than 16, the secondprediction parameter determination section 55 determines the predictionparameter #45 by use of the basic parameter set in place of the reducedset RS.

With the arrangement, it is possible to determine the predictionparameter #55 without using the reduced set RS, in a case where thespatial correlation between the prediction parameters is small. It istherefore possible to suppress an amount of a process for determining aprediction parameter.

(Additional Matter 3′)

Moreover, it is possible to have an arrangement in which, in accordancewith a characteristic of an image in the macro block, the predictionparameter determination section 242 employs, as the basic parameter set,one of (i) a parameter set whose main direction is a horizontaldirection (as shown in (a) of FIG. 16) and (ii) a parameter set whosemain direction is a vertical direction (as shown in (b) of FIG. 16).

For example, the prediction parameter determination section 242 canemploy, as the basic parameter set, the parameter set whose maindirection is the horizontal direction (as shown in (a) of FIG. 16), in acase where there is a horizontal edge in the macro block. On the otherhand, the prediction parameter determination section 242 can employ, asthe basic parameter set, the parameter set whose main direction is thevertical direction (as shown in (b) of FIG. 16), in a case where thereis a vertical edge in the macro block, for example.

Further, it is possible to have an arrangement in which (I), in a casewhere (i) a plurality of basic parameter sets are used selectively and(ii) the parameter set shown in (a) of FIG. 16 or the parameter setshown in (b) of FIG. 16 is selected as the basic parameter set, thesecond prediction parameter determination section 55 determines theprediction parameter #55 by use of the reduced set RS, and (II), in acase where the parameter set shown in FIG. 5 is selected as the basicparameter set, the second prediction parameter determination section 55decodes the prediction parameter set #55 by use of the basic parameterset in place of the reduced set RS.

With the arrangement, it is possible to determine whether to use thereduced set RS in accordance with a characteristic of an image in themacro block. It is therefore possible to reduce effectively an encodingamount of prediction parameters.

(Data Structure of Encoded Data #1)

The following description deals with a data structure of encoded data #1generated by the video encoding device 2, with reference to FIG. 15.

FIG. 15 is a view showing a bit stream structure of a bit stream #MBSfor each of a plurality of macro blocks of the encoded data #1. As shownin FIG. 15, the bit stream #MBS includes sub block information #SB1through #SBN (here, N is the number of sub blocks in the macro block),which is information related to sub blocks SB1 through SBN (here, N isthe number of sub blocks in the macro block) included in the macroblock.

Further, as shown in FIG. 15, each sub block information #SBn (1≦n≦N)includes (i) sub block position information #Ln which is informationindicating a position of a sub block SBn in the macro block, and (ii)prediction parameter information #Pn indicating a prediction parameterassociated with the sub block SBn.

The sub block position information #Ln is referred to in the videodecoding device for decoding the encoded data #1, so that the positionof the sub block SBn in the macro block is specified. Particularly, inthe video decoding device 1 described above, the sub block positioninformation #Ln is information which is referred to, so that the subblock SBn is classified into a group.

The prediction parameter information #Pn is information for specifying,in the video decoding device for decoding the encoded data #1, aprediction parameter associated with the sub block SBn. Particularly, inthe video decoding device 1 described above, the prediction parameterinformation #Pn is information indicating a prediction parameter amongprediction parameters which can be selected with respect to a group towhich the sub block SBn belongs.

For example, in a case where the sub block SBn belongs to the firstgroup, and the basic parameter set is a prediction parameter setconstituted by prediction parameters which can be selected with respectto the first group, the prediction parameter information #Pn isinformation indicating one of the prediction modes 0 through 8 includedin the basic parameter set.

Further, in a case where the sub block SBn belongs to the second group,the prediction parameter information #Pn is information indicating aprediction parameter among prediction parameters included in the reducedset RS.

Furthermore, in a case where the encoded data #1 is data generated byencoding, simultaneously, a prediction mode and a flag indicatingwhether or not the prediction parameter is identical with an estimatevalue, the prediction parameter #Pn can be represented by a code of ceil(log₂ (N−1)) bits. Here, N is the number of prediction parameters whichcan be selected with respect to the group to which the sub block SBnbelongs.

Generally, the number of prediction parameters included in the reducedset RS is smaller than the number of prediction parameters which can beselected with respect to the first group. Accordingly, an encodingamount of the prediction parameters included in the encoded data #1 issmall in a case where the reduced set RS is used, as compared with acase where the reduced set RS is not used.

Specifically, in a case where the encoded data #1 is data generated byencoding, simultaneously, a prediction mode and a flag indicatingwhether or not the prediction mode is identical with an estimate value,an encoding amount of the prediction parameter information #Pn issmaller by ceil (log₂ (Nfs−1))−ceil (log₂ (Nrs−1)) bits, as comparedwith a case where the reduced set RS is not used. Here, Nfs is thenumber of prediction parameters which can be selected with respect toeach of the plurality of sub blocks belonging to the first group, andNrs is the number of prediction parameters included in the reduced setRS.

As described above, by employing the reduced set RS, it is possible toreduce an encoding amount of the encoded data #1.

Note that the encoded data #1 may not include the sub block positioninformation #Ln. For example, by setting, in advance, a sub blockscanning order so that the sub block scanning order is used in both theencoding and the decoding in common, it becomes possible to determine aposition of a sub block on the basis of information indicating a numberof the sub block in the encoded data #1 in the sub block scanning order.

<Example of Application to Another Prediction Parameter>

In the above explanation, the prediction mode in the intra prediction ismainly used as the prediction parameter, as an example. Note, however,that the present invention is not limited to this. The present inventionis also applicable to another parameter generally used in generation ofa prediction image in an encoding process/decoding process of a video.

The following description deals with (i) an example in which the presentinvention is applied to a case where a motion vector in motioncompensation prediction is used as the prediction parameter and (ii) anexample in which the present invention is applied to a case where aweighting factor in luminance compensation prediction is used as theprediction parameter.

(Example of Application to Motion Vector)

In the motion compensation prediction, a position in a region on adecoded image used in prediction of a prediction target sub block isrepresented by a prediction parameter called “motion vector”.

The motion vector is selected from a prediction parameter set in whichthe number of elements depends on a size of an image and an accuracy(compensation accuracy). For example, in a case where (i) W is a width(the number of pixels in a horizontal direction) of an image, H is aheight (the number of pixels in a vertical direction) of the image, anda compensation accuracy of the image is 0.25 pixel, a motion vector V isselected from a prediction parameter set S defined by the followingformula.S≡{V|V=((N/4),(M/4))

(where: N is an integer satisfying an inequality of 0≦N<4W; and M is aninteger satisfying an inequality of 0≦M<4H).

In a case where the prediction parameter is such a motion vector, thefirst prediction parameter determination section 53 determines a motionvector for each of the plurality of sub blocks belonging to the firstgroup, and assigns the motion vector to the sub block. Further, thefirst parameter decoding section 43 can decode the motion vector foreach of the plurality of sub blocks belonging to the first group.

Each of the reduced set derivation sections 44 and 54 can generate thereduced set RS by use of with the motion vector assigned to each of theplurality of sub blocks belonging to the first group. Furthermore, eachof the reduced set derivation sections 44 and 54 can generate thereduced set RS by use of motion vectors assigned to sub blocks in thevicinity of the prediction target sub block, as described above.

As to each of the other sections of the video decoding device 1 and thevideo encoding device 2, it is possible to reduce an encoding amount ofthe prediction parameters included in the encoded data #1 by carryingout the same operations as those of the aforementioned arrangementemploying a prediction mode as the prediction parameter.

Note that, in a case where the prediction parameter is a motion vector,it is possible to have, for example, an arrangement in which each of thereduced set derivation sections 44 and 54 adds, to the reduced set RS,such a motion vector that a norm of a difference vector between themotion vector and a motion vector assigned to a sub block in thevicinity of the prediction target sub block is not more than a certainvalue.

Generally, in a case where the prediction parameter is a motion vector,the prediction parameter set includes a large number of predictionvectors. For example, the number of motion vectors included in theprediction parameter set S described above is 8000×4000 (W=2000,H=1000).

Meanwhile, the number of motion vectors accumulated in the reduced setRS has an upper limit which is equal to (i) the number of sub blocksbelonging to the first group, or (ii) the number of sub blocks in thevicinity of the prediction target sub block. For example, even in a casewhere the sub blocks included in the proximity sub block region NSRshown in (b) of FIG. 8 are used, the number of motion vectorsaccumulated in the reduced set RS is only 24.

Accordingly, depending on a characteristic of a target image of theencoding/decoding, the number of motion vectors accumulated in thereduced set RS might be not sufficient even in a case where the reducedset RS is generated only with the motion vectors assigned to the subblocks in the vicinity of the prediction target sub block. This mightcause a reduction in prediction accuracy, and an increase in encodingamount of residual data. By adding, to the reduction ser RS, such amotion vector that a norm of a difference vector between the motionvector and another motion vector assigned to a sub block located in thevicinity of the prediction target sub block is not more than a certainvalue, it is possible to reduce an encoding amount of the predictionparameters without having an increase in encoding amount of residualdata.

(Example of Application to Weighting Factor in Luminance CompensationPrediction)

In luminance compensation prediction, a luminance of the predictiontarget sub block is predicted by use of a value obtained by multiplying,by a weighting factor, each of a plurality of luminances of referencepictures, which are referred to in motion compensation prediction of aprediction target sub block.

The present invention can be applied to a case where the weightingfactor is used as a prediction parameter. For example, each of thereduced set derivation sections 44 and 54 can generate the reduced setRS on the basis of the weighting factor assigned to a sub block locatedin the vicinity of the prediction target sub block.

Note that, in a case where the present invention is applied to theweighting factor, (i) a plurality of values, each of which is used asthe weighting factor, may be associated with, respectively, a pluralityof representative values, a total number of which is set in advance, and(ii) the plurality of representative values may be used as predictionparameters.

For example, in a case where a value W of a weighting factor can be oneof real values (or values having a predetermined digit number)satisfying an inequality of 0≦W≦1, all values Wn of the weightingfactor, satisfying an inequality of (n/X)≦Wn≦((n+1)/X), are associatedwith representative values w, respectively, and the representativevalues w are used as prediction parameters. Here, X is a natural number,and n is an integer satisfying an inequality of 0≦n≦X−1. With suchassociation, all the real values satisfying an inequality of 0≦W≦1 canbe mapped in a set of representative values Wn, a total number of whichis X. Further, a specific value of X is set so that the encoding amountof the encoded data becomes small, for example.

As described above, by limiting the number of elements used asprediction parameters within a predetermined range, it becomes possibleto reduce the encoding amount of the encoded data, as compared with acase where the weighting factor itself is used as a predictionparameter.

(Additional Matters)

An image encoding device of the present invention, for encoding adifference between an input image and a prediction image, includes:classification means for (i) dividing a prediction image into aplurality of unit regions and (ii) classifying a plurality of predictionunits included in each of the plurality of unit regions into a firstgroup and a second group; first selection means for selecting, for eachof a plurality of prediction units belonging to the first group, aprediction parameter designating how to generate a prediction image, thefirst selection means selecting the prediction parameter from a basicset constituted by predetermined prediction parameters; second selectionmeans for selecting, for each of a plurality of prediction unitsbelonging to the second group, a prediction parameter designating how togenerate a prediction image, the second selection means selecting theprediction parameter from a reduced set which (i) includes at least apart of the prediction parameter(s) selected by the first selectionmeans and (ii) is constituted by a prediction parameter(s), the numberof which is not more than the number of prediction parameters includedin the basic set; and prediction parameter encoding means for encoding(i) information indicating which one of prediction parameters isselected, for each of the plurality of prediction units belonging to thefirst group, by the first selection means and (ii) informationindicating which one of prediction parameters is selected, for each ofthe plurality of prediction units belonging to the second group, by thesecond selection means.

According to the image encoding device having the arrangement describedabove, for each of the plurality of prediction units belonging to thesecond group, a prediction parameter designating how to generate aprediction image is selected from the reduced set which (i) includes atleast a part of the prediction parameter(s) selected by the firstselection means for each of the plurality of prediction units belongingto the first group which is included in the unit region in which thesecond group is included, and (ii) is constituted by a predictionparameter(s), the number of which is not more than the predictionparameters included in the basic set. The information indicating whichone of prediction parameters is selected by the second selection meansis encoded.

Here, generally, a prediction parameter with respect to each of theplurality of prediction units has a correlation with a predictionparameter with respect to another prediction unit located in thevicinity of the prediction unit. For this reason, the predictionparameter selected with respect to each of the plurality of predictionunits belonging to the first group is highly likely to be an optimumprediction parameter with respect to each of the plurality of predictionunits belonging to the second group. That is, the parameter selectedfrom the reduced set is highly likely to be an optimum predictionparameter with respect to each of the plurality of prediction unitsbelonging to the second group. Accordingly, with the arrangementdescribed above, it is possible to encode a prediction parameter withouthaving a reduction in encoding efficiency.

Further, with the arrangement described above, the reduced set includesat least a part of the prediction parameter(s) selected by the firstselection means, and is constituted by prediction parameters, the numberof which is not more than the number of prediction parameters includedin the basic set. Accordingly, for each of the plurality of predictionunits belonging to the second group, it is possible to have a reductionin encoding amount of information indicating which one of predictionparameters has been selected.

According to the arrangement described above, it is therefore possibleto have a reduction in encoding amount of information designating aprediction parameter, without having a reduction in encoding efficiency.

Furthermore, it is preferable that the reduced set is constituted byonly prediction parameters that (i) are selected by the first selectionmeans and (ii) are different from each other.

According to the arrangement, it is possible to further reduce thenumber of prediction parameters included in the reduced set.Accordingly, it is possible to further reduce an encoding amount. Morespecifically, prediction parameters generally have a spatial correlationwith each other, so that distribution of optimum prediction parameterswith respect to a plurality of prediction units in a certain regionbecomes inhomogeneous. For this reason, it is highly likely that, in thecertain region, only a part of various prediction parameters included inthe basic set is used. Accordingly, the number of prediction parametersin a certain region, selected by the first selection means, is highlylikely to be smaller than the number of prediction parameters includedin the basic set. For this reason, by employing, as the reduced set, agroup of all the prediction parameters which (i) are selected by thefirst selection means and (ii) are different from each other, it ispossible to reduce the number of elements of the reduced set.

Moreover, it is preferable that each of the plurality of unit regions isa unit used in encoding carried out by the image encoding device.

According to the arrangement, the generation process of the reduced setis carried out only once per encoding unit (e.g., each macro blockdefined in the H.264/MPEG-4 AVC standard). Accordingly, it is possibleto reduce an amount of the process for generating the reduced set.

Further, the image encoding device of the present invention ispreferably arranged such that the plurality of prediction unitsbelonging to the first group and the plurality of prediction unitsbelonging to the second group are arranged to form a checkered pattern(checker board pattern).

Generally, a prediction parameter with respect to each of a plurality ofprediction units has a correlation with a prediction parameter withrespect to another prediction unit located in the vicinity of theprediction unit.

According to the arrangement, the plurality of prediction unitsbelonging to the first group and the plurality of prediction unitsbelonging to the second group are arranged to form the checkered pattern(checker board pattern). Accordingly, a prediction parameter selectedwith respect to each of the plurality of prediction units belonging tothe first group is highly likely to be an optimum prediction parameterwith respect to each of the plurality of prediction units belonging tothe second group.

According to the arrangement, it is thus possible to reduce an encodingamount necessary for the encoding of a prediction parameter, withoutsacrificing encoding efficiency.

Furthermore, each of the prediction parameters can be a predictionparameter designating a prediction mode in intra prediction.

According to the arrangement, it is possible to reduce an encodingamount necessary for encoding of a prediction mode in intra prediction,without sacrificing encoding efficiency.

Moreover, it is preferable that the reduced set (i) includes theprediction parameter(s) selected by the first selection means and (ii)includes at least one of a vertical direction prediction mode in theintra prediction, a horizontal direction prediction mode in the intraprediction, and a DC prediction mode in the intra prediction, and, foreach of the plurality of prediction units belonging to the second group,the second selection means selects, from the reduced set, the predictionparameter designating how to generate a prediction image.

Generally, the vertical direction prediction mode, the horizontaldirection prediction mode, and the DC prediction mode are highlyfrequently selected in the intra prediction.

According to the arrangement, it is possible that the second selectionmeans selects, for each of the plurality of prediction units belongingto the second group, a prediction parameter designating how to generatea prediction image, and the second selection means selects theprediction parameter from the reduced set which (i) includes theprediction parameter(s) selected by the first selection means and (ii)includes one of the vertical direction prediction mode in the intraprediction, the horizontal direction prediction mode in the intraprediction, and the DC prediction mode in the intra prediction.Accordingly, it is possible to reduce an encoding amount without havinga reduction in prediction accuracy in generation of a prediction imagefor each of the plurality of prediction units belonging to the secondgroup.

Further, it is possible that the second selection means selects theprediction parameter in such a manner that (i), in a case where thenumber of a plurality of prediction units in each of the plurality ofunit regions, constituted by the plurality of prediction units belongingto the first group and the plurality of prediction units belonging tothe second group, is not less than a predetermined threshold value, thesecond selection means selects a prediction parameter from the reducedset, and (ii), in a case where said number is less than thepredetermined threshold value, the second selection means selects aprediction parameter from the basic set.

According to the arrangement, it is possible to select a predictionparameter from the basic set in a case where the number of predictionunits in the unit region is less than the predetermined value. It istherefore possible to reduce an amount of a process for selecting aprediction parameter.

The image encoding device of the present invention can be arranged suchthat the basic set is capable of being set per unit region, and thesecond selection means selects the prediction parameter in such a mannerthat (i), in a case where the basic set, which is set with respect toeach of the plurality of unit regions, constituted by the plurality ofprediction units belonging to the first group and the plurality ofprediction units belonging to the second group, satisfies a specificcondition, the second selection means selects a prediction parameterfrom the reduced set, and (ii), in a case where the basic set does notsatisfy the specific condition, the second selection means selects aprediction parameter from the basic set.

According to the arrangement, it is possible that the basic set iscapable of being set per unit region, and the second selection meansselects the prediction parameter in such a manner that (i), in a casewhere the basic set, which is set with respect to each of the pluralityof unit regions, constituted by the plurality of prediction unitsbelonging to the first group and the plurality of prediction unitsbelonging to the second group, satisfies a specific condition, thesecond selection means selects a prediction parameter from the reducedset, and (ii), in a case where the basic set does not satisfy thespecific condition, the second selection means selects a predictionparameter from the basic set. Accordingly, it is possible to reduce anencoding amount while reducing an amount of a process for selecting aprediction parameter.

Moreover, an image encoding device of the present invention can beexpressed as an image encoding device for encoding a difference betweenan input image and a prediction image, including: selection means forselecting, for each of a plurality of prediction units, a predictionparameter designating how to generate a prediction image, the selectionmeans selecting the prediction parameter from a reduced set including atleast a part of a prediction parameter(s) designating how to generate aprediction image(s) for a prediction unit(s) which (i) is located in thevicinity of a corresponding one of the plurality of prediction units and(ii) has been encoded; and prediction parameter encoding means forencoding, for each of the plurality of prediction units, informationindicating which one of prediction parameters has been selected by theselection means.

Generally, a prediction parameter with respect to each of a plurality ofprediction units has a correlation with a prediction parameter withrespect to another prediction unit located in the vicinity of theprediction unit. Accordingly, the reduced set is highly likely toinclude an optimum prediction parameter in generation of a predictionimage of each of the plurality of prediction units. Further, the reducedset is constituted by at least a part of the prediction parameter(s)with respect to the prediction unit(s) which is located in the vicinityof a corresponding one of the plurality of prediction units.Accordingly, the number of prediction parameters included in the reducedset is smaller than the number of prediction parameters included in aparameter set constituted by prediction parameters with respect toprediction units other than the each of the plurality of predictionunits.

According to the image encoding device of the present invention, havingthe arrangement described above, it is therefore possible to generateencoded data whose encoding amount is small, without sacrificingencoding efficiency.

Further, it is preferable that the prediction parameter encoding meansemploys, as a code indicating which one of prediction parameters isselected by the second selection means, a code having a length shorterthan that of a code indicating which one of prediction parameters isused by the first selection means.

According to the arrangement, prediction parameter encoding means canemploy, as a code indicating which one of prediction parameters isselected by the second selection means, a code having a length shorterthan that of a code indicating which one of prediction parameters isused by the first selection means. Accordingly, it is possible toencode, for each of the plurality of prediction units belonging to thesecond group, by use of the code having a short length, informationindicating which one of prediction parameters is selected.

Furthermore, it is preferable that the reduced set (i) includes theprediction parameter(s) selected by the first selection means, and (ii)is constituted by a prediction parameter(s), the number of which is2^(n)+1 (n is a natural number), and, for each of the plurality ofprediction units belonging to the second group, the second selectionmeans selects, from the reduced set, the prediction parameterdesignating how to generate a prediction image.

Generally, in a case where a group of elements, the number of which is2^(n)+1 (n is a natural number), is encoded, compression efficiency isimproved as compared with a case where a group of elements, the numberof which is not 2^(n)+1, is encoded.

According to the arrangement, it is possible to encode, for each of theplurality of prediction units belonging to the second group, informationindicating which one of prediction parameters is selected from thereduced set that (i) includes the prediction parameter(s) selected bythe first selection means, and (ii) is constituted by predictionparameters, the number of which is 2^(n)+1 (n is a natural number).Accordingly, it is possible to have an additional effect of enhancingcompression efficiency in encoding a prediction parameter.

Further, an image decoding device of the present invention, for decodingencoded data which is obtained in such a manner that (i) a differencebetween an original image and a prediction image is encoded for each ofa plurality of prediction units, and, simultaneously, (ii) selectioninformation is encoded, the selection information indicating which oneof a plurality of prediction parameters, each designating how togenerate a prediction image, is selected for each of the plurality ofprediction units, includes: classification means for classifying aplurality of prediction units included in each of a plurality of unitregions constituting the prediction image into a first group and asecond group; first selection means for selecting, for each of theplurality of prediction units belonging to the first group, a predictionparameter designating how to generate a prediction image, the firstselection means selecting, by referring to selection information foreach of a plurality of prediction units belonging to the first group,the prediction parameter from a basic set constituted by predeterminedprediction parameters; and second selection means for selecting, foreach of the plurality of prediction parameters belonging to the secondgroup, a prediction parameter designating how to generate a predictionimage, the second selection means selecting, by referring to selectioninformation for each of a plurality of prediction units belonging to thesecond group, the prediction parameter from a reduced set which (i)includes at least a part of the prediction parameter(s) selected by thefirst selection means and (ii) is constituted by a predictionparameter(s), the number of which is not more than the number of thepredetermined prediction parameters included in the basic set.

According to the image decoding device having the arrangement describedabove, it is possible to select, for each of the plurality of predictionunits belonging to the second group, a prediction parameter designatinghow to generate a prediction image, from the reduced set which (i)includes at least a part of the prediction parameter(s) selected by thefirst selection means for each of the plurality of prediction unitsbelonging to the first group which is included in the unit region towhich the second group belongs, and (ii) is constituted by a predictionparameter(s), the number of which is not more than the number ofprediction parameters included in the basic set.

Here, generally, a prediction parameter with respect to each of theprediction units has a correlation with a prediction parameter withrespect to another prediction unit located in the vicinity of theprediction unit. Accordingly, the prediction parameter selected for eachof the plurality of prediction units belonging to the first group ishighly likely to be an optimum prediction parameter with respect to eachof the plurality of prediction units belonging to the second group. Withthe arrangement described above, it is therefore possible to decode,without having a reduction in encoding efficiency, the predictionparameter from selection information having a reduction in encodingamount.

Furthermore, it is preferable that the reduced set includes onlyprediction parameters that (i) are selected by the first selection meansand (ii) are different from each other.

According to the arrangement, it is possible to further reduce thenumber of prediction parameters included in the reduced set.Accordingly, it is possible to further reduce an encoding amount.

Moreover, it is preferable that each of the plurality of unit regions isa unit used in decoding carried out by the image decoding device.

According to the arrangement, it is possible to carry out a process forgenerating the reduced set only once per decoding unit (e.g., each macroblock defined in the H. 264/MPEG-4 AVC standard). Accordingly, it ispossible to reduce an amount of the process for generating the reducedset.

Further, the image decoding device of the present invention ispreferably arranged such that the plurality of prediction unitsbelonging to the first group and the plurality of prediction unitsbelonging to the second group are arranged to form a checkered pattern(checker board pattern).

Generally, a prediction parameter with respect to each of a plurality ofprediction units has a correlation with a prediction parameter withrespect to another prediction unit located in the vicinity of theprediction unit.

According to an image encoding device having an arrangementcorresponding to the arrangement described above, the plurality ofprediction units belonging to the first group and the plurality ofprediction units belonging to the second group are arranged to form thecheckered pattern (checker board pattern). Accordingly, it is possibleto select an optimum prediction parameter with respect to each of theplurality of prediction units belonging to the second group. Accordingto the image encoding device having the arrangement corresponding to thearrangement described above, it is therefore possible to reduce anencoding amount necessary for the encoding of a prediction parameter,without sacrificing encoding efficiency.

According to the image decoding device having the arrangement describedabove, it is possible to decode the encoded data whose encoding amountis reduced in the manner described above.

Furthermore, the image decoding device of the present invention can bearranged such that each of the prediction parameters is a predictionparameter designating a prediction mode in intra prediction.

According to the arrangement described above, it is possible to decode,without sacrificing encoding efficiency, such encoded data that, in theintra prediction, an encoding amount of prediction modes is reduced.

Moreover, it is preferable that the reduced set (i) includes theprediction parameter(s) selected by the first selection means and (ii)includes at least one of a vertical direction prediction mode in theintra prediction, a horizontal direction prediction mode in the intraprediction, and a DC prediction mode in the intra prediction, and foreach of the plurality of prediction units belonging to the second group,the second selection means selects, from the reduced set, the predictionparameter designating how to generate a prediction image.

Generally, the vertical direction prediction mode, the horizontaldirection prediction mode, and the DC prediction mode are highlyfrequently selected in the intra prediction.

According to an image encoding device having an arrangementcorresponding to the arrangement described above, it is possible thatthe second selection means selects, for each of the plurality ofprediction units belonging to the second group, a prediction parameterdesignating how to generate a prediction image, and the second selectionmeans selects the prediction parameter from the reduced set which (i)includes the prediction parameter(s) selected by the first selectionmeans and (ii) includes one of the vertical direction prediction mode inthe intra prediction, the horizontal direction prediction mode in theintra prediction, and the DC prediction mode in the intra prediction.Accordingly, it is possible to reduce an encoding amount without havinga reduction in prediction accuracy in generation of a prediction imagefor each of the plurality of prediction units belonging to the secondgroup.

According to the image decoding device having the arrangement describedabove, it is possible to decode the encoded data whose encoding amountis reduced in the manner described above.

Further, the image decoding device of the present invention can bearranged such that the second selection means selects the predictionparameter in such a manner that (i), in a case where the number of aplurality of prediction units in each of the plurality of unit regions,constituted by the plurality of prediction units belonging to the firstgroup and the plurality of prediction units belonging to the secondgroup, is not less than a predetermined threshold value, the secondselection means selects a prediction parameter from the reduced set, and(ii), in a case where said number is less than the predeterminedthreshold value, the second selection means selects a predictionparameter from the basic set.

According to the arrangement, it is possible to select a predictionparameter from the basic set in a case where the number of predictionunits in the unit region is less than the predetermined value. It istherefore possible to reduce an amount of a process for selecting aprediction parameter.

Furthermore, the image decoding device of the present invention can bearranged such that the basic set is capable of being set per unitregion, and the second selection means selects the prediction parameterin such a manner that (i), in a case where the basic set, which is setwith respect to each of the plurality of unit regions, constituted bythe plurality of prediction units belonging to the first group and theplurality of prediction units belonging to the second group, satisfies aspecific condition, the second selection means selects a predictionparameter from the reduced set, and (ii), in a case where the basic setdoes not satisfy the specific condition, the second selection meansselects a prediction parameter from the basic set.

According to the arrangement, it is possible that the basic set iscapable of being set per unit region, and the second selection meansselects the prediction parameter in such a manner that (i), in a casewhere the basic set, which is set with respect to each of the pluralityof unit regions, constituted by the plurality of prediction unitsbelonging to the first group and the plurality of prediction unitsbelonging to the second group, satisfies a specific condition, thesecond selection means selects a prediction parameter from the reducedset, and (ii), in a case where the basic set does not satisfy thespecific condition, the second selection means selects a predictionparameter from the basic set. Accordingly, it is possible to decode theencoded data whose encoding amount is reduced, while reducing an amountof a process for selecting a prediction parameter.

Moreover, an image decoding device of the present invention can beexpressed as an image decoding device for decoding encoded data which isobtained in such a manner that (i) a difference between an input imageand a prediction image is encoded for each of a plurality of predictionunits, and, simultaneously, (ii) selection information is encoded, theselection information indicating which one of a plurality of predictionparameters, each designating how to generate a prediction image, isselected for each of the plurality of prediction units, including:selection means for selecting, for each of a plurality of predictionunits, a prediction parameter designating how to generate a predictionimage, the selection means selecting the prediction parameter from areduced set including at least a part of a prediction parameter(s)designating how to generate a prediction image(s) for a predictionunit(s) which (i) is located in the vicinity of a corresponding one ofthe plurality of prediction units and (ii) has been decoded.

Generally, a prediction parameter with respect to each of a plurality ofprediction units has a correlation with a prediction parameter withrespect to another prediction unit located in the vicinity of theprediction unit. Accordingly, the reduced set is highly likely toinclude an optimum prediction parameter in generation of a predictionimage of each of the plurality of prediction units. Further, the reducedset is constituted by at least a part of the prediction parameter(s)with respect to the prediction unit(s) which is located in the vicinityof a corresponding one of the plurality of prediction units.Accordingly, the number of prediction parameters included in the reducedset is smaller than the number of prediction parameters included in aparameter set constituted by prediction parameters with respect toprediction units other than the each of the plurality of predictionunits.

According to an image encoding device having an arrangementcorresponding to the arrangement described above, it is thereforepossible to generate encoded data whose encoding amount is small,without sacrificing encoding efficiency.

The image decoding device having the arrangement described above candecode the encoded data whose encoding amount is small as describedabove.

Further, it is preferable that the reduced set (i) includes theprediction parameter(s) selected by the first selection means, and (ii)is constituted by prediction parameters, the number of which is 2^(n)+1(n is a natural number), and, for each of the plurality of predictionunits belonging to the second group, the second selection means selects,from the reduced set, the prediction parameter designating how togenerate a prediction image.

Generally, in a case where a group of elements, the number of which is2^(n)+1 (n is a natural number), is encoded, compression efficiency isimproved as compared with a case where a group of elements, the numberof which is not 2^(n)+1, is encoded.

According to the arrangement described above, it is possible to decodethe encoded data having such high compression efficiency.

Furthermore, a data structure of the present invention, for encoded datawhich is obtained in such a manner that (i) a difference between aninput image and a prediction image is encoded for each of a plurality ofprediction units, and, simultaneously, (ii) selection information isencoded, the selection information indicating which one of a pluralityof prediction parameters, each designating how to generate a predictionimage, is selected for each of the plurality of prediction units,includes: selection information which is referred to in an imagedecoding device for decoding the encoded data, so as to select, for eachof the plurality of prediction units, a prediction parameter designatinghow to generate a prediction image, the prediction parameter beingselected from a reduced set which includes at least a part of aprediction parameter(s) designating how to generate a predictionimage(s) for a prediction unit(s) which (i) is located in the vicinityof a corresponding one of the plurality of prediction units and (ii) hasbeen decoded.

Generally, a prediction parameter with respect to each of a plurality ofprediction units has a correlation with a prediction parameter withrespect to another prediction unit located in the vicinity of theprediction unit. Accordingly, the reduced set is highly likely toinclude an optimum prediction parameter in generation of a predictionimage of each of the plurality of prediction units. Further, the reducedset is constituted by at least a part of the prediction parameter(s)with respect to the prediction unit(s) which is located in the vicinityof a corresponding one of the plurality of prediction units.Accordingly, the number of prediction parameters included in the reducedset is smaller than the number of prediction parameters included in aparameter set constituted by prediction parameters with respect toprediction units other than the each of the plurality of predictionunits.

Accordingly, the encoded data having the structure described above isthe encoded data whose encoding amount is reduced without sacrificingencoding efficiency.

The present invention is not limited to the description of theembodiments above, but may be altered by a skilled person within thescope of the claims. An embodiment based on a proper combination oftechnical means disclosed in different embodiments is encompassed in thetechnical scope of the present invention.

INDUSTRIAL APPLICABILITY

The present invention is suitably applicable to an image encoding devicefor generating encoded data by encoding an image, and an image decodingdevice for decoding the encoded data generated by such an image encodingdevice.

REFERENCE SIGNS LIST

-   1: Video decoding device-   14: MB decoding section-   144: Prediction parameter decoding section-   41: Group determination section (classification means)-   42: Switch section-   43: First prediction parameter decoding section (first selection    means)-   44: Reduced set derivation section-   45: Second prediction parameter decoding section (second selection    means)-   2: Video encoding device-   24: MB encoding section-   242: Prediction parameter determination section-   51: Group determination section (classification means)-   52: Switch section-   53: First prediction parameter determination section (first    selection means)-   54: Reduced set derivation section-   55: Second prediction parameter determination section (second    selection means)-   243: Prediction parameter encoding section (prediction parameter    encoding means)-   248: Prediction image generation section

The invention claimed is:
 1. An image decoding device for decodingencoded data which is obtained in such a manner that (i) a differencebetween an input image and a prediction image is encoded for each of aplurality of prediction units, and (ii) a prediction parameter used togenerate the prediction image is encoded, the image decoding devicecomprising: prediction parameter decoding means for decoding aprediction parameter which is to be applied to each of the plurality ofprediction units; reduced set creating means for creating a reduced setwhich is a group of prediction parameters; and second predictionparameter decoding means for (i) decoding a first prediction parameterwhich specifies a prediction parameter included in a basic set or (ii)decoding a second prediction parameter from a bit sequence having alength determined in accordance with the number of prediction parametersincluded in the reduced set, the second prediction parameter specifyinga prediction parameter included in the reduced set, the reduced setincluding at least one prediction parameter included in a predictionunit region in the vicinity of a target prediction unit selected fromthe plurality of prediction units, the number of prediction parametersincluded in the reduced set being less than the number of predictionparameters included in the basic set which is constituted bypredetermined prediction parameters, the prediction parameter decodingmeans carrying out decoding, in accordance with a value of a flag whichis included, per prediction unit, in the encoded data, so that (i) theprediction parameter is decoded by use of the first prediction parameterdecoded by the second prediction parameter decoding means or (ii) theprediction parameter is decoded by use of the second predictionparameter decoded by the second prediction parameter decoding means, andin a case where the prediction parameters which are identical with eachother exist in at least two prediction parameters included in theprediction unit region in the vicinity of the target prediction unit,the prediction parameter decoding means causing the reduced set toinclude one of the prediction parameters which are identical with eachother, and carrying out decoding, per prediction unit, the at least oneprediction parameter included in the reduced set.
 2. The image decodingdevice as set forth in claim 1, wherein: each of the predictionparameters is a motion vector used in motion compensation prediction. 3.The image decoding device as set forth in claim 1, said secondprediction parameter decoding means, in accordance with the value of theflag, (i) decoding a first prediction parameter which specifies aprediction parameter included in a basic set or (ii) decoding a secondprediction parameter from a bit sequence having a length determined inaccordance with the number of prediction parameters included in thereduced set, the second prediction parameter specifying a predictionparameter included in the reduced set.
 4. An image encoding device forencoding a difference between an input image and a prediction image,comprising: prediction parameter encoding means for encoding aprediction parameter which is applied to each of a plurality ofprediction units; reduced set creating means for creating a reduced setwhich is a group of prediction parameters; and second predictionparameter determination means for (i) determining a first predictionparameter which specifies a prediction parameter included in a basic setor (ii) determining a second prediction parameter which specifies aprediction parameter included in a reduced set, the reduced setincluding at least one prediction parameter included in a predictionunit region in the vicinity of a target prediction unit selected fromthe plurality of prediction units, the number of prediction parametersincluded in the reduced set being less than the number of predictionparameters included in a basic set, the prediction parameter encodingmeans encoding, per prediction unit, a flag for designating that (i) theprediction parameter is encoded by use of the first prediction parameterdetermined by the second prediction parameter determination means or(ii) the prediction parameter is encoded by use of the second predictionparameter determined by the second prediction parameter determinationmeans, and in a case where the prediction parameters which are identicalwith each other exist in at least two prediction parameters included inthe prediction unit region in the vicinity of the target predictionunit, the prediction parameter encoding means causing the reduced set toinclude one of the prediction parameters which are identical with eachother, and carrying out encoding, per prediction unit, the at least oneprediction parameter included in the reduced set.
 5. The image encodingdevice as set forth in claim 4, said second prediction parameterdetermination means, in accordance with the value of the flag, (i)determining a first prediction parameter which specifies a predictionparameter included in a basic set or (ii) determining a secondprediction parameter which specifies a prediction parameter included ina reduced set.